[CMake] Unable to change CMAKE_CXX_FLAGS for vc6

[Andrew MacDonald]

I have been unable to change the setting of CMAKE_CXX_FLAGS added to a
vc6 project file. The other variables such as BUILD_INCLUDES and
EXTRA_DEFINES work properly. I've tried both of the following

set(CMAKE_CXX_FLAGS "foo")

set(CMAKE_CXX_FLAGS "foo" CACHE STRING "" FORCE)

The latter changes the cache value but still doesn't change the value
in the generated project file. Does anyone know how to do this
properly?

Thanks,
Andrew