[Cmake] getting GCC version

[Samson Timoner]

I have some code that when compiled using GCC, requires gcc 3.0 or greater.
Once it is known in the cmake file that the compiler is GCC, is there a
relatively easy way to get the GCC version number?

I'd like CMAKE to spit out an error message if the GCC version is too low.

 Thanks in advance,

-- Samson