[CMake] [MSVC] Setting warning level on target feels like long-time bug

[Brad King]

On 12/9/18 8:09 AM, Marc CHEVRIER wrote:
> The real question is how to manage cleanly target specific flags
> overriding global or directory defaults?

All the optimization and warning flags currently handled by
`CMAKE_<LANG>_FLAGS[_<CONFIG>]` need to have abstractions
introduced (e.g. target properties or something) and the
defaults re-thought.  Transition can be handled via a policy.

-Brad