[CMake] Adding C(XX)FLAGS by using an environment variable?

[Mike Arthur]

I'm wanting to try and add C(XX)FLAGS by using an environment variable so I 
can pass "-fmessage-length=0" only when compiling inside Eclipse.

Is there any way to do this? CMake doesn't seem to respect C(XX)FLAGS as 
environment variables at "make" time.
-- 
Cheers,
Mike Arthur