Make Xcode debug info format controllable through a qmake variable

The default is still DWARF instead of DWARF with dSYM for static builds
of Qt, so that debug builds of the final application don't take forever
to build due to generating the dSYM file.

Change-Id: I370d800d7c959e05c1a8780c4ebf58fff250daa1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>