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.