cmake HAVE_DECL_NAN examples

Geocld/PeaSyo third-party/json-c/CMakeLists.txt :163 

check_symbol_exists(nan         "math.h" HAVE_DECL_NAN)

ByConity/ByConity contrib/jsonc-cmake/CMakeLists.txt :173 

check_symbol_exists(nan         "math.h" HAVE_DECL_NAN)