Set Java version from property instead of by plugin configuration.
Set Java version from property instead of by plugin configuration.

This makes it somewhat easier to override the Java version from
child POMs.
Also, it does not nail down the maven-compiler-plugin version
anymore. Sure enough, this bumped the version from 2.3.1 to 2.3.2
(2.3.2 fixes http://jira.codehaus.org/browse/MCOMPILER-109).

mkaring/nifty-gui,Teencrusher/nifty-gui,Ryszard-Trojnacki/nifty-gui,void256/nifty-gui,void256/nifty-gui,atomixnmc/nifty-gui,atomixnmc/nifty-gui,bgroenks96/nifty-gui,gouessej/nifty-gui,xranby/nifty-gui,Ryszard-Trojnacki/nifty-gui,bgroenks96/nifty-gui,Teencrusher/nifty-gui,relu91/nifty-gui