qmake: Update internal mkspec on changes to QMAKESPEC

Allows the macx-xcode mkspec to be a wrapper around other mkspecs.

Since QMAKESPEC can now be set in the spec, we have to ensure not
to append to QMAKESPEC.

Change-Id: Idf33ff38147f14c488f14b426c02d9a739fdaecf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/a9bad65b091d15b0446fa141e0a69ebb94b38b70)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>