[CMake] Removing default COMPILE_FLAGS

[Daniel Nelson]

> I believe CMAKE_CXX_FLAGS is consulted each time an add_library() or
> add_executable() call is processed. So you should be able to remove the
> flag, add the singleton library/executable, and put the flag back.
> 
> tyler

This is the first thing I tried, however it doesn't seem to work.  
I seems they are written to the flags.make files at the end, 
with whatever the final value of CMAKE_CXX_FLAGS is.
--
Daniel