cmake HAVE_ISNAN_IN_CMATH examples

llvm-dcpu16/llvm-dcpu16 cmake/config-ix.cmake :103 

check_symbol_exists(isnan cmath HAVE_ISNAN_IN_CMATH)

KhronosGroup/SPIRV-LLVM cmake/config-ix.cmake :140 

check_symbol_exists(isnan cmath HAVE_ISNAN_IN_CMATH)