It seems that MinGW-w32 started distributing GDI+ headers since this
version and MinGW-w64 might have supported them for even longer, but
it's difficult to test for the MinGW distribution used in this header,
as it is included before wx/msw/gccpriv.h which defines
__MINGW32_TOOLCHAIN__ and __MINGW64_TOOLCHAIN__ symbols and changing
this is tricky due to relative order of defining UNICODE and
wxUSE_UNICODE and including MinGW headers, which can only be included
once UNICODE is set properly.
But while the fully correct solution is difficult, just checking for the
compiler version should solve the problem in 99.99% of the cases in
practice as there should be vanishingly few people using MinGW-w64 with
gcc < 4.8 currently, so this simple solution is good enough.
Closes#17973.
