Make compiler flags system dependent

I added the -Wno-unused-local-typedefs flag to cmake/modules/FindXenomai.cmake to suppress a bunch of warnings that started showing up in Ubuntu 14.04. This flag does not apply on the 12.04 system, and it causes a warning cc1plus: warning: unrecognized command line option "-Wno-unused-local-typedefs" [enabled by default]. Find a way to only use this flag on systems that need it.