Don't scan system includes for dependencies and don't list them

Assume that C and C++ headers found in system paths will not change,
so we don't need to tell Make about them, nor do we need to scan their
contents either.

The previous qmake behavior matched gcc's -M switch; it now matches
the -MM switch:

       -M  Instead of outputting the result of preprocessing, output a
           rule suitable for make describing the dependencies of the
           main source file.

       -MM Like -M but do not mention header files that are found in
           system header directories, nor header files that are
           included, directly or indirectly, from such a header.

This goes hand-in-hand with our use of -isystem to pass system paths
to the compiler.

Change-Id: I3346b6da496fe6495ac89c5286d066b343116f0e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>