Store the GCC version number in Q_CC_GNU

The sequence of (__GNUC__ * 100 + __GNUC_MINOR__) was used in quite a
few places. Simplify it to make the code more readable.

This follows the change done for Clang, which was quite necessary since
Apple's version of Clang has different build numbers.

Change-Id: I886271a5a5f21ae59485ecf8d140527723345a46
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>