qmake: fix const correctness

Partial cherry-pick of
https://qt.gitorious.org/qt/jpnurmi-qt/commit/8c4ef19

Task-number: QTBUG-21910
Change-Id: Ieb833a977fc00d2637f8419278698c82b6086e2f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
qmake: remove bogus comment

the respective code was removed in 375edf7

Change-Id: Ie31ef4bc8970b5396f50f1c4963f378df816242a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>