forrest requires a running X-server on UNIX

When the "forrest" command is used to compile the site on a server machine that does not run a graphical display (no X server running), it fails with the stacktrace below. When an X server is (temporarily) started, it works.

<snip>
Lazy mode: true
Lazy mode: true
Lazy mode: true
* [1/20] [20/20] 9.222s 7.2Kb linkmap.html
Lazy mode: true
Lazy mode: true
* [2/19] [0/0] 0.631s 1.0Kb skin/print.css
Lazy mode: true
* [3/18] [0/0] 8.943s 0b skin/images/rc-b-l-15-1body-2menu-3menu.png
Exception in thread "main" java.lang.InternalError: Can't connect to X11 window server using 'localhost:0.0' as the value of the DISPLAY variable.
        at sun.awt.X11GraphicsEnvironment.initDisplay(Native Method)
        at sun.awt.X11GraphicsEnvironment.access$000(X11GraphicsEnvironment.java:53)
        at sun.awt.X11GraphicsEnvironment$1.run(X11GraphicsEnvironment.java:142)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.awt.X11GraphicsEnvironment.<clinit>(X11GraphicsEnvironment.java:131)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:164)
        at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:68)
        at java.awt.image.BufferedImage.createGraphics(BufferedImage.java:1141)
        at org.apache.batik.ext.awt.image.GraphicsUtil.createGraphics(GraphicsUtil.java:515)
        at org.apache.batik.gvt.filter.GraphicsNodeRed8Bit.genRect(GraphicsNodeRed8Bit.java:120)
        at org.apache.batik.gvt.filter.GraphicsNodeRed8Bit.copyData(GraphicsNodeRed8Bit.java:108)
        at org.apache.batik.ext.awt.image.rendered.TileCacheRed.genRect(TileCacheRed.java:53)
        at org.apache.batik.ext.awt.image.rendered.AbstractTiledRed.drawBlockInPlace(AbstractTiledRed.java:594)
        at org.apache.batik.ext.awt.image.rendered.AbstractTiledRed.drawBlock(AbstractTiledRed.java:527)
        at org.apache.batik.ext.awt.image.rendered.AbstractTiledRed.copyToRasterByBlocks(AbstractTiledRed.java:420)
        at org.apache.batik.ext.awt.image.rendered.AbstractTiledRed.copyData(AbstractTiledRed.java:287)
        at org.apache.batik.ext.awt.image.rendered.TranslateRed.copyData(TranslateRed.java:97)
        at org.apache.batik.ext.awt.image.rendered.PadRed.copyData(PadRed.java:87)
        at org.apache.batik.gvt.renderer.StaticRenderer.repaint(StaticRenderer.java:390)
        at org.apache.batik.gvt.renderer.StaticRenderer.repaint(StaticRenderer.java:340)
        at org.apache.batik.transcoder.image.ImageTranscoder.transcode(ImageTranscoder.java:105)
        at org.apache.batik.transcoder.XMLAbstractTranscoder.transcode(XMLAbstractTranscoder.java:132)
        at org.apache.cocoon.serialization.SVGSerializer.notify(SVGSerializer.java:206)
        at org.apache.cocoon.xml.dom.SVGBuilder.endDocument(SVGBuilder.java:131)
        at org.apache.cocoon.xml.AbstractXMLPipe.endDocument(AbstractXMLPipe.java:55)
        at org.apache.cocoon.components.sax.XMLTeePipe.endDocument(XMLTeePipe.java:67)
        at org.apache.xml.serializer.ToXMLSAXHandler.endDocument(ToXMLSAXHandler.java:180)
        at org.apache.xalan.transformer.TransformerImpl.transformNode(TransformerImpl.java:1287)
        at org.apache.xalan.transformer.TransformerImpl.run(TransformerImpl.java:3383)
        at org.apache.xalan.transformer.TransformerHandlerImpl.endDocument(TransformerHandlerImpl.java:389)
        at org.apache.cocoon.xml.AbstractXMLPipe.endDocument(AbstractXMLPipe.java:55)
        at org.apache.cocoon.transformation.TraxTransformer.endDocument(TraxTransformer.java:562)
        at org.apache.xml.serializer.ToXMLSAXHandler.endDocument(ToXMLSAXHandler.java:180)
        at org.apache.xalan.transformer.TransformerImpl.transformNode(TransformerImpl.java:1287)
        at org.apache.xalan.transformer.TransformerImpl.run(TransformerImpl.java:3383)
        at org.apache.xalan.transformer.TransformerHandlerImpl.endDocument(TransformerHandlerImpl.java:389)
        at org.apache.cocoon.xml.AbstractXMLPipe.endDocument(AbstractXMLPipe.java:55)
        at org.apache.cocoon.transformation.TraxTransformer.endDocument(TraxTransformer.java:562)
        at org.apache.cocoon.environment.internal.EnvironmentChanger.endDocument(EnvironmentStack.java:118)
        at org.apache.cocoon.xml.AbstractXMLPipe.endDocument(AbstractXMLPipe.java:55)
        at org.apache.cocoon.components.sax.XMLTeePipe.endDocument(XMLTeePipe.java:67)
        at org.apache.cocoon.components.sax.XMLByteStreamInterpreter.parse(XMLByteStreamInterpreter.java:83)
        at org.apache.cocoon.components.sax.XMLByteStreamInterpreter.deserialize(XMLByteStreamInterpreter.java:73)
        at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.processXMLPipeline(AbstractCachingProcessingPipeline.java:215)
        at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.process(AbstractProcessingPipeline.java:664)
        at org.apache.cocoon.components.source.impl.SitemapSource.toSAX(SitemapSource.java:349)
        at org.apache.cocoon.components.source.SourceUtil.parse(SourceUtil.java:206)
        at org.apache.cocoon.generation.FileGenerator.generate(FileGenerator.java:117)
        at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.processXMLPipeline(AbstractCachingProcessingPipeline.java:234)
        at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.process(AbstractProcessingPipeline.java:428)
        at org.apache.cocoon.components.treeprocessor.sitemap.SerializeNode.invoke(SerializeNode.java:134)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.ContainerNode.invoke(ContainerNode.java:38)
        at org.apache.cocoon.components.treeprocessor.CategoryNode.invokeByName(CategoryNode.java:66)
        at org.apache.cocoon.components.treeprocessor.sitemap.CallNode.invoke(CallNode.java:83)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.sitemap.SelectNode.invoke(SelectNode.java:87)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.ContainerNode.invoke(ContainerNode.java:38)
        at org.apache.cocoon.components.treeprocessor.CategoryNode.invokeByName(CategoryNode.java:66)
        at org.apache.cocoon.components.treeprocessor.sitemap.CallNode.invoke(CallNode.java:83)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:54)
        at org.apache.cocoon.components.treeprocessor.sitemap.PreparableMatchNode.invoke(PreparableMatchNode.java:112)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:138)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:95)
        at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:203)
        at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:149)
        at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:248)
        at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:112)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:54)
        at org.apache.cocoon.components.treeprocessor.sitemap.PreparableMatchNode.invoke(PreparableMatchNode.java:112)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:138)
        at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:76)
        at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:95)
        at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:203)
        at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:149)
        at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:248)
        at org.apache.cocoon.Cocoon.process(Cocoon.java:503)
        at org.apache.cocoon.bean.CocoonWrapper.getPage(CocoonWrapper.java:456)
        at org.apache.cocoon.bean.CocoonBean.processTarget(CocoonBean.java:501)
        at org.apache.cocoon.bean.CocoonBean.process(CocoonBean.java:362)
        at org.apache.cocoon.Main.main(Main.java:321)