Since commit 230ea218 (OS X: Improve default CMAKE_OSX_SYSROOT
selection, 2012-09-21) we always set CMAKE_OSX_SYSROOT if any SDK is
found in order to support Makefile generator builds with Xcode >= 4.3
without the command-line tools installed.  However, in the basic
POSIX-only case of the Makefile generator with command-line tools and no
CMAKE_OSX_ARCHITECTURES we should not select any SDK by default.