[CMake] Interaction between source COMPILE_FLAGS and directory COMPILE_OPTIONS

[Braden McDaniel]

The documentation for the source property COMPILE_FLAGS describes it 
as, "Additional flags to be added when compiling this source file."  
However, when using the Visual Studio 2013 generator (at least), it appears 
to replace (rather than complement) the options added via the directory 
property COMPILE_OPTIONS.

It (i.e., source COMPILE_FLAGS) also replaces options added by setting the 
CMAKE_<LANG>_FLAGS variable.

Is this the intended behavior?

Braden