Add check for cmake build system so that if the OpenCL library is not found, it will not compile the OpenCL-dependent code (on APPLE)

Thanks to Max for the report and fix in Issue 480.

src/BulletMultiThreaded/GpuSoftBodySolvers/OpenCL/CMakeLists.txt

@@ -11,6 +11,6 @@ IF(BUILD_NVIDIA_OPENCL_DEMOS)
 	SUBDIRS(NVidia)
 ENDIF()
 
-IF(APPLE)
+IF(APPLE AND OPENCL_LIBRARY) 
 	SUBDIRS(Apple)
 ENDIF()