[CMake] CMake 3.6 and OSX

[Robert Ramey]

I've just "upgraded" to version 3.6 of CMake.  I'm using Xcode with the 
clang compiler.  Now when I'm trying to configure a project I'm getting:

The C compiler identification is unknown
The CXX compiler identification is unknown
CMake Error at CMakeLists.txt:14 (project):
   No CMAKE_C_COMPILER could be found.
CMake Error at CMakeLists.txt:14 (project):
   No CMAKE_CXX_COMPILER could be found.

What do I have to do to make this work?

Robert Ramey