AX_PROG_EMACS: New submission

* m4/ax_prog_emacs.m4: New file.

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+# ===========================================================================
+#      https://www.gnu.org/software/autoconf-archive/ax_prog_emacs.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PROG_EMACS
+#
+# DESCRIPTION
+#
+#   This macro allows the end user to specify a particular Emacs executable
+#   via a configure script command-line arg. For example:
+#
+#     ./configure EMACS=$HOME/build/GNU/emacs/src/emacs
+#
+#   It also arranges to mention env var EMACS in the './configure --help'
+#   output. See info node "(autoconf) Generic Programs" for details.
+#
+#   More precisely...
+#
+#   If env var EMACS is set, try to use its value directly, but avoid
+#   getting fooled by value 't' (set by older Emacsen for subprocesses). If
+#   no joy from the environment, search for "emacs" via AC_CHECK_PROG. If
+#   still no joy, display "Emacs not found; required!" and make configure
+#   exit failurefully. Otherwise, set shell var EMACS and AC_SUBST it, too.
+#
+# LICENSE
+#
+#   Copyright (c) 2016-2017 Thien-Thi Nguyen
+#
+#   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 3 of the License, 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/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 1
+
+AC_DEFUN([AX_PROG_EMACS],[dnl
+AC_ARG_VAR([EMACS],[Use this Emacs to byte-compile the Emacs Lisp files.])
+dnl Allow env override but do not get fooled by ‘EMACS=t’.
+test t = "$EMACS" && unset EMACS
+dnl The next line does nothing if var ‘EMACS’ is already set.
+AC_CHECK_PROG([EMACS], [emacs], [emacs])
+AS_IF([test "x$EMACS" = x],[AC_MSG_ERROR([Emacs not found; required!])])
+])dnl
+
+dnl ax_prog_emacs.m4 ends here