The "cmakexbuild" wrapper is not needed for Xcode 4 and above, and the
path to it may change when CMake moves. Avoid storing a specific path
to a build program in CMakeCache.txt and instead compute the value for
CMAKE_MAKE_PROGRAM on demand. However, if a user does set the value
explicitly then honor it.
This does for Xcode what commit v3.0.0-rc1~260^2~4 (VS: Switch to
internal CMAKE_MAKE_PROGRAM lookup by generators, 2013-11-15) did for
Visual Studio generators.
Brad King