Clean up how we build against SDKs on Mac OS

Instead of setting -isysroot in both arch.test, compile.test, the various
mkspecs, and sdk.prf, we now propgate the chosen SDK as the qmake
variable QMAKE_MAC_SDK, which is then handled exclusivly in sdk.prf.

The QMAKE_MAC_SDK variable, and -sdk argument to configure, is expected
to be of the short-form name, eg macosx or iphoneos, not a full path, as
that's what Xcode also expects. We take care of translating that into
a full path for -isysroot/-syslibroot in sdk.prf, using xcodebuild as
a helper.

Change-Id: I281655b2fa5180c6e78ffdce36824e4a91447570
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>