Integrate gcov support into Qt build system.

To instrument a Qt application or library with the gcov coverage
tool, do `CONFIG+=gcov' in the application .pro file.

To instrument Qt itself with gcov, use the `-gcov' configure
option.

Change-Id: If24e91d95318609b0df1a76ed6d679bd92bcaab2
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>