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.

HACKING

@@ -55,7 +55,7 @@ follows.
 * Updating auxilary scripts
 
   Fetch new versions of the files that are maintained in other GNU
-  repositories by running "cd build-aux; ./gnu-fetch".  In case any file in the
+  repositories by running "make fetch".  In case any file in the
   Mcron repository has been updated, commit and re-run the testsuite.
 
 * Code coverage

build-aux/gnu-fetch

@@ -1,97 +0,0 @@
-#!/bin/sh
-# Fetch files maintained in other GNU repositories.
-
-scriptversion=2018-03-24.21; # UTC
-
-# Copyright © 2018 Mathieu Lirzin <mthl@gnu.org>
-#
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation; either version 2, or (at your option)
-# any later version.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program.  If not, see <https://www.gnu.org/licenses/>.
-
-set -e
-
-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/gitlog-to-changelog
-  ${sv_git_gl}build-aux/do-release-commit-and-tag
-  ${sv_git_gl}build-aux/gnu-web-doc-update
-  ${sv_git_gl}build-aux/gnupload
-"
-
-usage="Usage: $0
-
-fetch files maintained in other GNU repositories
-"
-
-while test -n "$1"
-do
-  case $1 in
-  -*)
-    case $1 in
-    --help)
-      echo "$usage"
-      exit $?
-      ;;
-    --version)
-      echo "gnu-fetch $scriptversion"
-      exit $?
-      ;;
-    --)
-      shift
-      break
-      ;;
-    -*)
-      echo "$0: Unknown option '$1', try '$0 --help'" 1>&2
-      exit 1
-      ;;
-    esac
-    ;;
-  esac
-  shift
-done
-
-rm -rf Fetchdir
-mkdir Fetchdir
-for url in ${FETCHFILES}
-do
-  file=`printf '%s\n' "$url" | sed 's|^.*/||; s|^.*=||'`
-  "$WGET" -nv "$url" -O "Fetchdir/$file" || exit 1
-  if cmp "Fetchdir/$file" "$file" >/dev/null; then
-      : Nothing to do
-  else
-    echo "$0: updating file $file"
-    cp "Fetchdir/$file" "$file" || exit 1
-  fi
-done
-rm -rf Fetchdir
-
-exit 0
-
-# Local variables:
-# eval: (add-hook 'before-save-hook 'time-stamp)
-# time-stamp-start: "scriptversion="
-# time-stamp-format: "%:y-%02m-%02d.%02H"
-# time-stamp-time-zone: "UTC0"
-# time-stamp-end: "; # UTC"
-# End:

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