qt5base: support OVERRIDE_SRCDIR

With OVERRIDE_SRCDIR we don't apply any of the qt5base patches, but the
custom specs files are needed to be able to build - So install these in the
configure step instead of having them as a patch.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>