Add macros for enabling/disabling warnings

This allows us to avoid the ugly #if for compiler versions. We might
still need for when a warning only occurs in one compiler version, but
otherwise the code will be much cleaner.

Change-Id: Ibc941d898b3dad2e3d87c11378f29139c31f0fff
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>