Add script to build xcode 4.6 project files

.. just upgraded to Xcode 4.6 and the project files
created by qmake 4.8 don't work. After scanning the
web I found a workaround in QTBUG-29371 and implemented
it in a little shell script: qmake-xcode.

.. to run, from the src directory invoke it with
$ sh ./qmake-xcode