Make better use of GNU Pth -- patch by Andy Dustman. I can't test this, so I'm just checking it in with blind faith in Andy. I've tested that it doesn't broeak a non-Pth build on Linux. Changes include: - There's a --with-pth configure option. - Instead of _GNU_PTH, we test for HAVE_PTH. - Better signal handling. - (The config.h.in file is regenerated in a slightly different order.)