This was already done for clang -Winconsistent-missing-override, but
gcc has a similar warning since 5.1 and, moreover, latest versions of
clang support this gcc warning as well, so add a special macro which
handles both compilers and use it in all wx macros defining virtual
functions instead of just disabling one of the clang warnings.
Closes https://github.com/wxWidgets/wxWidgets/pull/2000
c924ecb10a