qmake: Expose qmake arguments as QMAKE_ARGS

Allows project files or mkspecs to call qmake recursively using system()
with the right arguments, which we use to fix the ios default_post.prf.

Change-Id: I90d69e2b156bb0f0af1279188b11f81c84c24fb8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/1cdbe4752bef972db5471e811e9a781749cd0380)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>