Fix detection of mingw gcc >=4.6

Make sure that the version check works for gcc e.g. from mingw-builds,
which contains spaces in the version info, e.g.

g++ (i686-posix-dwarf, Built by MinGW-W64 project) 4.8.2

In Qt 5, support for MinGW-gcc versions older than 4.6 was dropped.

Change-Id: Idac5fb3a8cd01200558032ea1bd997e30f0e8b8b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Remove incorrect check for mingw 64 bit

MinGW-w64 has '64' too, but isn't necessarily a 64 bit build ...
But the variable wasn't used anyway, so we can as well just remove
the check.

Change-Id: Ifba3ce344c5dc5e692f105bc99081ae4c69c779c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Fix the Carbon build

The function does not exist when building for Carbon with the 10.5 SDK
so since this means the function will effectively do nothing, so we
just return false in this case.

Change-Id: If41762a055ebeccfcfce751e164b6a6e7e1db437
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>