Allow eclipse plugin to take compiler versions from maven.compiler.source/target properties

I'd like that the maven eclipse plugin could detect the source and target compiler versions from the properties maven.compiler.source and maven.compiler.target in addition to the explicit declaration given in the compiler plugin configuration.

I submit a patch that seems to fix the issue (though I did not attached any junit test).

The patch proceeds like this:

• first, it checks the usual <source> and <target> configurations.
• if either is not found, it try to get the configuration from the maven.compiler.source and maven.compiler.target properties.
• if not found, and the compiler plugin version >= 2.3, then it defaults to 1.5 (as stated in the compiler plugin docs)

More info:
http://mail-archives.apache.org/mod_mbox/maven-users/201109.mbox/%3CCAKY3QyXbNMHHktYD3XB%3DTe9hEY3Firg2mLzQZ7LF7CFsJ3qWsw%40mail.gmail.com%3E