[CMake] /W1 and resource.rc files

[Gonzalo Garramuño]

I have a VC++ project which compiles with ninja.  I add all the sources 
and a rc file into a single executable by placing them in a variable.
I am running into trouble specifying the /W1 flag in add_definitions.
The flag gets passed onto RC and it fails its compilation.
Also, I get warnings of:
cl : Command line warning D9025 : overriding '/W3' with '/W1'
I am unsure what sets /W3.

-- 
Gonzalo Garramuño