fix sandbox build - use of <profile> for conditional not working for child projects

The SandboxDistr has common, factored-out build steps for the sandbox projects. One of this is to conditionally run the docbook build for the child project, if that project has a build_documentation.xml file.

This was being done using a <profile> element in the factored-out Sandbox pom, which activated if that file was present.
This works when you do mvn install from the child project; the <profile> is inherited via the <parent> chain, and all is well.

However, if you run from the SandboxDistr (with the intent of running the build recurisvely on all the <module> children projects, this fails - because the <profile> is evaluated just once, for the SandboxDistr project.

Fix this by abandoning this method of conditional execution, and reverting to <ant> conditional tests.