Use QMAKE_GCC_MAJOR_VERSION to check GCC version for extensions

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>

configure.pri

@@ -314,6 +314,6 @@ defineTest(qtConfTest_hasThumbFlag) {
 }
 
 defineTest(qtConfTest_hasGcc6OrNewer) {
-    greaterThan(QT_GCC_MAJOR_VERSION, 5):return(true)
+    greaterThan(QMAKE_GCC_MAJOR_VERSION, 5):return(true)
     return(false)
 }