testing with _POSIX2_VERSION=200112 in the environment.

* BUGS: Suggest 'diff -C2', not 'diff -c2'.
* FAQ: Suggest 'sort -k 1.2', not 'sort +0.1'.
* depcomp: Sync to the depcomp shipped with Automake 1.7.8, as
it has the bug fixed and that's better than maintaining our
own depcomp.
(Patch from Paul Eggert <eggert@twinsun.com>.)