maint: Replace "build-aux/gnu-fetch" with 'fetch' maintainer rule

Fetching third-party files inside a make rule is convenient and less
error prone since 'make' has access to '$srcdir'.

* build-aux/gnu-fetch: Delete.
* maint.mk (WGET, git_sv_host, sv_git_am, sv_git_gl)
(fetchfiles): New macros.
(fetch): New target.
* HACKING <Updating auxilary scripts>: Update instructions.

maint.mk

@@ -18,3 +18,46 @@
 
 # Rebuild Makefile.in if this file is modifed.
 Makefile.in: maint.mk
+
+## -------------------- ##
+##  Third-party files.  ##
+## ---------------------##
+
+WGET = wget
+
+# Git repositories on Savannah.
+git_sv_host = git.savannah.gnu.org
+
+# Some repositories we sync files from.
+sv_git_am = 'https://$(git_sv_host)/gitweb/?p=automake.git;a=blob_plain;hb=HEAD;f='
+sv_git_gl = 'https://$(git_sv_host)/gitweb/?p=gnulib.git;a=blob_plain;hb=HEAD;f='
+
+# Files that we fetch and which we compare against.
+# Note that the 'lib/COPYING' file must still be synced by hand.
+fetchfiles = \
+  $(sv_git_am)contrib/test-driver.scm \
+  $(sv_git_gl)build-aux/do-release-commit-and-tag \
+  $(sv_git_gl)build-aux/gitlog-to-changelog \
+  ${sv_git_gl}build-aux/gnu-web-doc-update \
+  $(sv_git_gl)build-aux/gnupload
+
+# Fetch the latest versions of few scripts and files we care about.
+# A retrieval failure or a copying failure usually mean serious problems,
+# so we'll just bail out if 'wget' or 'cp' fail.
+fetch:
+	$(AM_V_at)rm -rf Fetchdir
+	$(AM_V_at)mkdir Fetchdir
+	$(AM_V_GEN)set -e; \
+	if $(AM_V_P); then wget_opts=; else wget_opts=-nv; fi; \
+	for url in $(fetchfiles); do \
+	   file=`printf '%s\n' "$$url" | sed 's|^.*/||; s|^.*=||'`; \
+	   $(WGET) $$wget_opts "$$url" -O Fetchdir/$$file || exit 1; \
+	   if cmp Fetchdir/$$file $(srcdir)/build-aux/$$file >/dev/null; then \
+	     : Nothing to do; \
+	   else \
+	     echo "$@: updating file $$file"; \
+	     cp Fetchdir/$$file $(srcdir)/build-aux/$$file || exit 1; \
+	   fi; \
+	done
+	$(AM_V_at)rm -rf Fetchdir
+.PHONY: fetch