[CMake] cache variables from compiler checks

[Clinton Stimpson]

I noticed something odd...

If I delete the cache variable CMAKE_STRIP, then re-run cmake, it doesn't come 
back.
If I delete the build/CMakeFiles directory, re-run cmake, it'll come back.

Is that intentional?

I have a dashboard build where I was re-configuring with a different initial 
CMakeCache.txt file, and some of the variables don't come back.  Looks like 
I'll also need to remove the CMakeFiles directory while I write the 
CMakeLists.txt file.

Clint