For Mac OS X we currently specify build tools without an absolute path,
which means we end up using the ones in /usr/bin. This is wrong, we
should be using the tools from the toolchain of the chosen SDK.

For iOS we do specify an absolute path, by resolving the toolchain
path in the iOS makespecs.

To solve the situation on Mac OS X, we move the logic of resolving the
toolchain path to sdk.prf, and share it between OSX and iOS.

For configure we need to duplicate some of the logic from sdk.prf, as
configure pulls out QMAKE_CC and QMAKE_CXX for running some initial
tests and building qmake. The new macSDKify function also solves
the issue of missing sysroot and deployment version in the flags.

Change-Id: Ib1d239c9904cf3ccee5214b313cf6205869a1462
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>