Add OS X Oracle JDK include paths

Since java.home is defined as pointing at the jre dir inside the
'actual' JAVA_HOME (e.g.
/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home), we
need to add include paths that point at the include dir outside the jre
dir.
Add missing OS X pthread.h include

If this include is not here, Callbacks.c will fail to build on
Mavericks, complaining about pthread_mutex_t being an unknown type.
Fix OpenAL include paths on OS X

OpenAL is a system framework on OS X, using Apple's own implementation,
so we can add extra include paths for that in the native POM. As a side
effect we need to drop the 'AL' prefix from the include paths.
Add missing return values in audio_start

Can't just 'return' nothing when the function has return type void *.