cmake HAVE__ISNAN examples

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

CHECK_FUNCTION_EXISTS(_isnan HAVE__ISNAN)

GenericMappingTools/gmt cmake/modules/ConfigureChecks.cmake :420 

check_symbol_exists (_isnan    "${_math_h}" HAVE__ISNAN)