cmake HAVE_HTTPLIB examples

Lewuathe/mlir-hello thirdparty/llvm-project/llvm/cmake/config-ix.cmake :254 

check_cxx_symbol_exists(CPPHTTPLIB_HTTPLIB_H ${HTTPLIB_HEADER_PATH} HAVE_HTTPLIB)

root-project/root interpreter/llvm-project/llvm/cmake/config-ix.cmake :218 

check_cxx_symbol_exists(CPPHTTPLIB_HTTPLIB_H ${HTTPLIB_HEADER_PATH} HAVE_HTTPLIB)