Add build info on Mac

.. see src/mac/Info.plist.app, based upon the default for
   Mac with clang found in mkspecs/default/Info.plist.app
   from the Qt installation directories.

.. we make our own to add build version information, which
   should be placed there via the qmake 'VERSION' macro but
   its not supported -- you have to edit the plist file instead.

Fixes #796