this is done by recognizing the -- option and putting everything that
follows it into the QMAKE_EXTRA_ARGS variable.

the purpose of this is being able to pass "alien" arguments (for example
configure options) without having to marshal them into a qmake variable
assignment manually. this is expected to greatly enhance the convenience
when no wrapper script (i.e., configure itself) is used (which will be
the case outside qtbase).

Change-Id: I47735bdab2811d17d7d394fbf638ebd14332ea73
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
(cherry picked from qtbase/26d44fce3dfb9dbd3b5be2a031fff8c1bb903960)
Reviewed-by: Jake Petroules <jake.petroules@qt.io>