[CMake] Support for Custom LANG in CMAKE (CUDA and general case)

[t m]

> As far as I correctly understood patch from Peter, implementation for
> makefile dependency is on place.
> For me makefile dependency is fine, since I'm using only makefiles.
>
> I just tried to understand idea of LANGUAGE inside cmake. I think that
> in this particular case having new language on place is really good
> idea. This is what for language was.

I will also vote for this solution. Since there is no difference in
the workflow why to use separate macros and separate implementation.
Why to not simply reuse LANG interface and C/CPP dependency scanner.
The same analogy is in case of C and CXX - there is no
distinguish like: c_add_executable and cxx_add_executable.

regards, Tomasz Majchrowski.