Clarify in docs that MacOS is no longer a supported platform

[gwbrown]

Whatever the official goals of this project, it is clear that brl-cad, in its current state is not usable on MacOS and should not be advertised as such. To give some concrete evidence here:

• No MacOS binaries have been provided for any recent version. In fact, the most recent version to have pre-built MacOS binaries was 7.24.0, released over a decade ago.
• Builds fail on M1 Macs
• Builds fail on mac in general - there was supposedly a fix coming for this one, unclear if it ever landed.
• Even if you can build a binary, it fails immediately upon startup.

Only one of these issues has any response from a maintainer, and it was "hey I'm working on a fix for this", with no follow-up. The upshot is that BRL-CAD IS NOT USABLE ON MACOS CURRENTLY, HASN'T BEEN FOR A DECADE, AND THERE SEEMS TO BE NO PLAN FOR MAKING IT SO.

I'm mostly writing this in the hopes of preventing other people from wasting a bunch of time trying to get this working - if you want to try out brl-cad, just fire up a Linux VM. It doesn't work on Mac, despite what is claimed.