[CMake] GCC version variable

[Emmanuel Blot]

Is there some CMake macro to report the version of the GCC compiler  
version?
It seems these macros exist for the CL compiler, but I've not found  
the matching macros for GCC.

Some warnings are produced w/ GCC 4, but the matching warning shut-off  
option only exists for GCC 4 (not for GCC3).
I'd like to enable the -Wno-... option switch only for build built  
with gcc 4, and do nothing with GCC 3.

Thanks,
Manu