The thing is, 20 years ago maybe you developed on a Sun box, then the guy down the hall tried to compile it on his HP machine and it crapped itself because of some Sun-related assumption you made in setting things up. Then you mail it to a colleague across the state and he compiles it on OSF/1, hurrah!
But now, people write Linux libraries, tested on a Linux box, for people running Linux, and only compiled on Linux. There may be OS X support. That's it. This is not the 1980s, where 10,000 Unixes bloomed. With 4 Makefiles, you could support Linux, OS X, Windows, and FreeBSD, and thus cover 99% of your audience. I'm sick of watching configure scripts take longer to run than the actual compilation.
Except that before "just supporting Linux (and maybe MacOS)" was the thing to do, it was "just support Sun (and maybe HP-UX)". And before that, it was "just support BSD/Vax (and maybe SVR2)".
So, when you take a broader view than "just support what everyone uses", you're not just helping niche platforms -- you're future-proofing.
But now, people write Linux libraries, tested on a Linux box, for people running Linux, and only compiled on Linux. There may be OS X support. That's it. This is not the 1980s, where 10,000 Unixes bloomed. With 4 Makefiles, you could support Linux, OS X, Windows, and FreeBSD, and thus cover 99% of your audience. I'm sick of watching configure scripts take longer to run than the actual compilation.