[setup] Handle all mac versions > 10.5 the same way, since they provide
an utility named java_home to get the current JAVA_HOME. Added a little
helper method to check if the given java_home contains a 'jni.h', so it
is in fact a valid JDK. Also pretty printed errors
fixed idention error
Merge pull request #40 from marscher/reworked_jdk_detection_macosx

[setup] Handle all mac versions > 10.5 the same way, removed setting of library_dir since it is ignored by distutils