QMakeProjectManager: Support QMAKE_EXTRA_COMPILERS

Task-number: QTCREATORBUG-11212
Change-Id: I84358d6c2e9cb76eb72c8f85c6f7fedd2c4cfb0f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
QMakeNodes: OBJECTIVE_HEADERS doesn't exist

Change-Id: I788c018bfc8bb608ea82696618d4eb9e0d2fe918
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>