MacOSX: Add initial support in CMake build infra
Include the OpenAL headers using the "al.h" form, as recommended by its CMake finder script
Fix system_other implementation by adding an Init() and fixing the wrong const
Added option to force to use bundled gmock/gtest

 * resolves problem when version of gmock/gtest found installed in system
   disagrees with the bundled one
 * removed -DGTEST_HAVE_PTHREAD=0, seems that pthread is needed after all