cmake _CXX_COROUTINES_HAVE_HEADER examples

apple/foundationdb cmake/FindCoroutines.cmake :152 

check_include_file_cxx("coroutine" _CXX_COROUTINES_HAVE_HEADER)

apple/foundationdb cmake/FindCoroutines.cmake :169 

mark_as_advanced(_CXX_COROUTINES_HAVE_HEADER)