build: Support VPATH builds.

This allows using 'mcron' before it is installed without hardcoding the
local build environment in the executable.

* build-aux/pre-inst-env.in: New script.
* configure.ac (AC_CONFIG_FILES): Create it.
(AC_CONFIG_HEADER): Add 'config.h'.
(moduledir): New variable.
(PACKAGE_LOAD_PATH): new C preprocessor macro.
* mcron.c: Include "config.h".
(main): Don't overwrite Guile load paths.
(inner_main): Prepend Mcron modules load paths.
* Makefile.am (.scm.go, doc/mcron.1): Use 'pre-inst-env'.
(mcron_CFLAGS): Remove GUILE_LOAD_PATH macro.
(noinst_SCRIPTS): New variable.
* .gitignore: Update.

build-aux/pre-inst-env.in

@@ -0,0 +1,35 @@
+#!/bin/sh
+
+# Copyright © 2015, 2016 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/>.
+
+abs_top_srcdir="`cd "@abs_top_srcdir@" > /dev/null; pwd`"
+abs_top_builddir="`cd "@abs_top_builddir@" > /dev/null; pwd`"
+
+GUILE_LOAD_COMPILED_PATH="$abs_top_builddir/scm${GUILE_LOAD_COMPILED_PATH:+:}$GUILE_LOAD_COMPILED_PATH"
+GUILE_LOAD_PATH="$abs_top_builddir/scm:$abs_top_srcdir/scm${GUILE_LOAD_PATH:+:}:$GUILE_LOAD_PATH"
+export GUILE_LOAD_COMPILED_PATH GUILE_LOAD_PATH
+
+PATH="$abs_top_builddir:$PATH"
+export PATH
+
+# Define $MCRON_UNINSTALLED to prevent 'mcron' from prepending @moduledir@ to
+# the Guile load paths.
+MCRON_UNINSTALLED=1
+export MCRON_UNINSTALLED
+
+exec "$@"