build: Add "maint.mk"

* maint.mk: New maintainer-only Makefile fragment.
* Makefile.am: Include it.
* configure.ac: Substitute "maint.mk" content when it exists.

Makefile.am

@@ -257,3 +257,9 @@ guilec_verbose_0 = @echo "  GUILEC  " $@;
 devnull_verbose = $(devnull_verbose_@AM_V@)
 devnull_verbose_ = $(devnull_verbose_@AM_DEFAULT_V@)
 devnull_verbose_0 = >/dev/null
+
+## ------------- ##
+## Maintenance.  ##
+## ------------- ##
+
+@MAINT_MAKEFILE@

configure.ac

@@ -139,6 +139,12 @@ AC_ARG_WITH(tmp-dir,
 AC_MSG_RESULT($CONFIG_TMP_DIR)
 AC_SUBST(CONFIG_TMP_DIR)
 
+# Include the Maintainer's Makefile fragment, if it's here.
+MAINT_MAKEFILE=/dev/null
+AS_IF([test -r "$srcdir/maint.mk"],
+  [MAINT_MAKEFILE="$srcdir/maint.mk"])
+AC_SUBST_FILE([MAINT_MAKEFILE])
+
 AC_CONFIG_FILES([pre-inst-env:build-aux/pre-inst-env.in],
   [chmod +x pre-inst-env])
 AC_CONFIG_FILES([doc/config.texi

maint.mk

@@ -0,0 +1,20 @@
+## Maintainer-only Makefile fragment
+# Copyright © 2018 Mathieu Lirzin <mthl@gnu.org>
+#
+# This file is part of GNU Mcron.
+#
+# GNU Mcron 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 3 of the License, or
+# (at your option) any later version.
+#
+# GNU Mcron 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 GNU Mcron.  If not, see <http://www.gnu.org/licenses/>.
+
+# Rebuild Makefile.in if this file is modifed.
+Makefile.in: maint.mk