Went and hit the SourceForge compile farm. XPLC compiles and runs fine on sparc64-unknown-linux, i686-pc-linux, i386-unknown-freebsd4.7, alphaev67-unknown-linux, armv4l-unknown-linux, with at most some minor glitches.
Red Hat is annoying me a bit, because a workaround for a bug in gcc is now unneeded, and I'll have to write an autoconf test for that. No biggie, but it won't compile because of that.
Mac OS X and Solaris are giving me more trouble. Both seem to have different way of handling sonames (maybe they don't support them at all, but I think I saw stuff related to that in man pages), and Mac OS X annoyingly doesn't support __attribute((weak)), but I don't see any way I could test for that or make it good. I might have to lose that weak reference trick, but that would be a shame, it is really nice when it works.