cmake HAVE__ISNAN examples

Froser/gamemachine src/3rdparty/openal-soft-1.19.1/CMakeLists.txt :615 

CHECK_FUNCTION_EXISTS(_isnan HAVE__ISNAN)

EvEmu-Project/evemu_Crucible CMakeLists.txt :140 

CHECK_CXX_SYMBOL_EXISTS( _isnan   "cfloat" HAVE__ISNAN )