At least on my system (Archlinux with a qt5.git super-build),
QT_GCC_MAJOR_VERSION seems to be unset when the test is run, causing it to fail
and extensions to be disabled (despite using GCC 8).

Change-Id: I384c6ebbbbe7245f41d014cd2f7aeca2fec73073
Reviewed-by: Michael Brüning <michael.bruning@qt.io>