Mac: Set minimum version to 10.7 for clang-libc++

Mac: Set minimum version to 10.7 for clang-libc++

Clang's libc++ does not support 10.6.

Add mac-minimum-version.conf which sets the version
to 10.6. Set the version to 10.7 in the clang-libx++*
mkspecs.

Change-Id: I494d0d24b0d73d9395e9d5406c8c63c9af87f8cc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>