d/configure: Prepare for execution on other OS

[smoe]

Somehow I feel like it may possibly not be completely pointless to consider a compilation of LinuxCNC or parts of it on non-Linux Operating Systems. At least we should prepare debian/configure for the event that someone is trying it. Here is what I came up with. My personal immediate motivation is to support writers of the documentation who are likely to have a the source tree on whatever OS their machine may be.

My attempt on MacOS did not complete a configure run. But that is just because of missing packages in brew - nothing technical. Conda may also be worthwhile to have a look, but the libgpiod-dev equivalent for either I fail to find. It may be of interest to come up with a configure option that prepared for the documentation only.

[rene-dev]

Afaik it does build and run on freebsd, and on macos I got stuck getting the linker to create hal modules.

[andypugh]

I did look into this and MacOS does have some form of RT API. But I think that Rene said is wasn't useful for us.
https://developer.apple.com/library/archive/documentation/Darwin/Conceptual/KernelProgramming/scheduler/scheduler.html

[smoe]

At the very moment, starting of with what brew provides, LinuxCNC does not configure on MacOS. I do not know where any sort of direction for users of other platforms should be gatherning, so I must admit. And d/configure should eventually be removed in favour of a distribution-independent setup, I guess. So this PR is a bit of an experimental nature. I happily change it any way you want, am just uncertain if that URL should be in the d/configure sources or rather in some new section of our documentation - or maybe there should be a forum section for porting LinuxCNC.