Use check_symbol_exists() instead of check_function_exists().

This is mostly a revert of 477031c03b2c525b117fae8a9b61c98245a06908.
Now that we are not setting `CMAKE_C_FLAGS`, the check can work
correctly and it helps `pcc` (portable c compiler) make it further
along--though it still doesn't produce usable results (see #427 for the
details).