Disable automatic use of pch for MinGW

Revert cb09e1e88944e1ab9b0d0 for MinGW. gcc on Windows reproducably crashes
when the pre-compiled header becomes big enough ...

Change-Id: Icd5a3dfbe59f5ff5c78832e7b4436d0f1cfa1031
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>