depcomp bug fixed (but not prettily)

[Jim Meyering <meyering at ascend dot com>]

I've just checked in a change that finally lets me use the head
of the CVS automake module to build Makefile.in files for fileutils,
textutils, and the sh-utils.

Before the change to automake.in,

	* automake.in (handle_dependencies): Set $require_file_found{'depcomp'}
	if the depcomp file exists, before calling require_config_file on
	`depcomp'.  This makes require_file_internal skip its buggy existence
	test that would make automake fail (with `required file `lib/depcomp'
	not found') when AC_CONFIG_AUX_DIR is not set.

I was getting the above error about lib/depcomp.
Specifying AC_CONFIG_AUX_DIR was not an option because I have config
files in all of `.', src, and doc.

I've added a test that fails with the old version of automake in two ways.
First, it made automake fail outright, complaining that lib/depcomp didn't
exist (true, but irrelevant).  I tweaked automake not to give that
failure, but still found depcomp being listed in lib/Makefile.in's definition
of DIST_COMMON.  That would induce a `make'-time failure.

My patch is pretty clean at least in that its changes are localized, though it
does abuse module boundaries a little in setting $require_file_found{depcomp}.
If anyone can find a better way to solve the problem, I hope you'll share
it with us.