BUG: VTK_JAVA_SOURCE_VERSION to accomodate different java versions

The variable can be set to the JDK version installed on the
machine to link against. Default is 1.5 to maintain backwards
compatibility for binary packages.

Change-Id: I4f721c5f8fcf6666971418ee97862d6ae905352b
Merge topic 'fix_java_warnings' into master

596243f2 BUG: VTK_JAVA_SOURCE_VERSION to accomodate different java versions