Scope cached Mac SDK tool values by mkspec

Otherwise doing stuff like -spec macx-g++ when the default spec is clang
will not have an effect on the tools used.

Change-Id: Ia2769abfdd8c19f79d427b9f09707430e736305a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Make the macx-xcode spec a wrapper around the default spec

... except for MAKEFILE_GENERATOR = XCODE. This means the spec no longer
hard-codes g++, and will work regardless of whether the default spec was
clang or g++.

This require us to set QMAKE_XCODE_GCC_VERSION properly for GCC, so that
additional compilation flags passed by Xcode will match the actual
compiler used.

Task-number: QTBUG-31713
Change-Id: If65140a7471cd16f483036742f1d5b86d0485c52
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>