[CMake] Adding cache entries to cmake gui

[Edward Diener]

Using the cmake gui program of the latest cmake 3.7.2 I add a cache 
entry of CMAKE_CXX_FLAGS to some compile flag value, but as soon as I 
hit the generate button the cache entry disappears and is no longer 
retained. Is CMAKE_CXX_FLAGS no longer a valid cmake variable ? If it is 
not a valid cmake variable, what cmake variable can I use to add a cpp 
compiler flag to my cmake build ?