Automatically update "aclocal_m4_deps" when updating gnulib

Automatically update "aclocal_m4_deps" when updating gnulib

When we update gnulib using our "update-gnulib.sh" tool, it doesn't
automatically update the list of M4 files present at
gnulib/Makefile.in:aclocal_m4_deps.  This patch extends the tool to do
that.  It also puts "aclocal_m4_deps" in its own file (a Makefile
fragment), so that it's easier to update it programatically.

Tested by generating the file and diff'ing the results against the
current version of "aclocal_m4_deps".

gdb/ChangeLog:
2018-09-04  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gnulib/Makefile.in (aclocal_m4_deps): Move to
	"aclocal-m4-deps.mk".  Include file here.
	$(srcdir)/aclocal.m4: Add "configure.ac".
	* gnulib/aclocal-m4-deps.mk: New file.
	* gnulib/update-gnulib.sh: Automatically update
	"aclocal-m4-deps.mk".