Make better use of GNU Pth -- patch by Andy Dustman.

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.)