qtbase: Add gcc to QMAKE_COMPILER variable.

qtbase: Add gcc to QMAKE_COMPILER variable.

According to mkspecs/features/spec_post.prf, QMAKE_COMPILER variable should at least contain gcc for linux-oe-g++ platform.

This patch fixes a crash issue in QtWebKit introduced in v5.4.0 by:
https://qt.gitorious.org/qt/qtwebkit/commit/34d4628b981add039a9a349723cbdff1dadca144

(From meta-qt5 rev: ad958ec94752371aa25bfbfd0f60489a20a45c0f)

Signed-off-by: Julien Brianceau <jbriance@cisco.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>