Drop the last use of CMAKE_OSX_SYSROOT_DEFAULT. Replace internal
platform variable CMAKE_${lang}_HAS_ISYSROOT with a more general
CMAKE_${lang}_SYSROOT_FLAG variable. If the -isysroot flag exists and
CMAKE_OSX_SYSROOT points to an SDK (not "/") then always add it to
compiler command lines. This is already done in the Xcode IDE.
Clang has the same interface as GNU except that we do not need to test
for the deployment target and sysroot flags. Simply set variables
CMAKE_${lang}_HAS_ISYSROOT
CMAKE_${lang}_OSX_DEPLOYMENT_TARGET_FLAG
to true because every version of Clang available on OS X supports these
flags.