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>