cmake HAVE_LOG2F examples

Froser/gamemachine src/3rdparty/openal-soft-1.19.1/CMakeLists.txt :557 

CHECK_SYMBOL_EXISTS(log2f  math.h HAVE_LOG2F)

GenericMappingTools/gmt cmake/modules/ConfigureChecks.cmake :403 

check_symbol_exists (log2f       "${_math_h}" HAVE_LOG2F)