typecheck-gcc: handle function pointers properly

All the callbacks passed to curl_easy_setopt are defined as function
pointers. The possibility to pass both functions and function pointers
was handled for the callbacks that typecheck-gcc.h defined as
compatible, but not for the public callback types themselves.

This makes all compatible callback types defined in typecheck-gcc.h
function pointers too and checks all functions uniformly with
_curl_callback_compatible, which handles both functions and function
pointers.

A symptom of the problem was a warning in tool_operate.c with
--disable-libcurl-option and without --enable-debug as that file
passes the callback functions to curl_easy_setopt directly.

Fixes https://github.com/curl/curl/issues/1403
Closes https://github.com/curl/curl/pull/1404