Fix disabling of aliasing for all gcc alike compilers

Don't rely on the makespec to contain g++ in its name, instead use
QMAKE_COMPILER, which gets added to CONFIG.

Fixes crashes on QNX.

Change-Id: If8d2a6a7aa9998b311b2ca137af2fa236adecb00
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>