[CMake] extending cmake with gm2 support

[Martin Kalbfuß]

Hi,

I'm using the gm2 compiler, which is a Modula-2 compiler. CMake has no
support for it. Currently, no build system supports it. So I decided to
take a look at CMake, and try to set up a library project.
But I haven't figured out how to introduce the compiler. What's the best
way to do this?

Many thanks,

Martin