Distinguish between 'mac' and 'macx' qmake scopes

The former applies both on Mac OS X and iOS, but 'macx' is specific to
Mac OS X.

ios.conf and macx.conf now share most of their settings in the common
mac.conf. We set the default QMAKE_MAC_SDK before loading mac.conf, so
that any overrides in the device config will apply afterwards. This
means configure's mkspec parsing will be able to read the QMAKE_MAC_SDK.

Change-Id: I0c7e26a6a0103e19b23ef152aa9e4ab461cee632
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>