Allow installing multiple versions of rgbds

[Rangi42]

make install could name its executables as rgbasm0.6.0, rgblink0.6.0, etc, and symlink rgbasm, rgblink, etc to the latest version. This would let older projects continue to use old rgbds without having to keep a local copy somewhere.

[ISSOtm]

Sounds more like a job for the distro packager.

This would not let old projects work without specifying e.g. make RGBASM=... RGBLINK=... explicitly, at which point you don't even need make install anyway.

[aaaaaa123456789]

I think it's a bit more conventional to do rgbasm-0.6.0, etc.

[mid-kid]

I agree this should be left up to the distro pacakger, but it'd be nice to have an upstream convention. Many autotools projects allow passing a --program-suffix=-$VERSION flag, to help with this.
Personally I'm torn between both installing the bins in a separate directory and using different names, might do both.

EDIT: I guess cmake allows a -DCMAKE_EXECUTABLE_SUFFIX=-$VERSION but I haven't tested this yet.