JavaCL: mark clCreateSubDevices as optional
JavaCL: fix typo: CLDevice.getOpenCLVersion -> getOpenCLCVersion
JavaCL: add CLPlatform.unloadPlatformCompiler() (already deprecated in 1.2)
JavaCL: introduce CLPlatform.requireMinVersionValue to deal with OpenCL version requirements.
JavaCL: support OpenCL 1.2 clEnqueueBarrierWithWaitList & clEnqueueMarkerWithWaitList through added CLQueue.enqueueBarrier & .enqueueMarker (switch between OpenCL 1.1 and OpenCL 1.2 variants automatically) (issue #232)