headers: Disable gcc warning that __INTRINSIC_PROLOG is not portable

If the mingw-w64 headers are included in a way that doesn't mute
warnings in system headers, this header causes lots of warnings like:
warning: this use of "defined" may not be portable

It is caused by the fact that this macro __INTRINSIC_PROLOG uses
non-portable Conditional inclusion  (ISO WG14 N2176 (C17) 6.10.1/4).

Disable this gcc warning via localized GCC pragmas to prevent 300 lines of
warnings for every application.

Signed-off-by: Martin Storsjö <martin@martin.st>