Fetch gnulib files from gitweb.

Fetch gnulib files from gitweb.

‘make fetch’ pulls the files maintained by Automake from gitweb, so we
always get the HEAD revision at the time of the operation.  The files
maintained by Gnulib, on the other hand, are just copied from whatever
local gnulib checkout you happen to have on your hard drive at the
time, which might not have been updated recently and might not even be
checked out from gnulib trunk.

This patch makes us use the same logic for Gnulib that we do for
Automake.  It also pulls all of the code out of cfg.mk to a separate
shell script.  The lists of files to update from each repository are
also moved to that file.

 * build-aux/fetch.sh: New script.  Lists of files to fetch from
   Gnulib and Automake are now kept here.
 * cfg.mk (fetch): Run fetch.sh, it does all the work.
   (gnulib-update, autom4te-update, WGETFLAGS, automake_gitweb)
   (autom4te_files, move_if_change): Delete.