[CMake] Recognizing GNU compiler doesn't work

[Niko Vuokko]

Simply put variable CMAKE_COMPILER_IS_GNUC is an empty string and thus false.

I'm using CMake 2.6.2, GCC 4.1.0 on Linux and doing a C project. I also tried 
putting an underscore there between GNU and C. The weird thing here is that 
when running CMake, it's first message is "The C compiler identification is 
GNU", but testing the variable myself later on gives out nothing.

Does anyone have ideas on debugging this?