Update build.xml

Automates sdk/jvm detection so that developers running new platforms can compile the native library. It shall be compatible with new upcoming releases such as ${os.version} = 10.10.x.

* We need to add ant-contrib.jar in the libs/ folder to enable this automation.
* Native dynamic libjawt.dylib, involved in the link task must be found in the libs/macosx folder too.

Regards,
@b23prodtm