dist: accept .md versions for README et al.

This change was suggested by madmurphy; some ideas were taken
from the patch he provided.
https://lists.gnu.org/archive/html/automake-patches/2021-06/msg00005.html

* bin/automake.in (@toplevelmd_ok): new global, listing the files
for which we will accept .md versions.
(@common_files): remove those files from there.
(handle_dist): check for .md if non-.md is absent.
(handle_minor_options): check for README-alpha.md.
(check_gnu_standards): accept .md version if present.
(check_gnits_standards): likewise.
(usage): output list of .md-accepted files.
* doc/automake.texi (Basics of Distribution): document.

* t/toplevelmd.sh: new test.
* t/list-of-tests.mk (handwritten_tests): add it.
* NEWS: mention new feature.

2 files changed, 70 insertions, 1 deletions

diff --git a/t/list-of-tests.mk b/t/list-of-tests.mk
index 51456ef51..934930b50 100644
--- a/t/list-of-tests.mk
+++ b/t/list-of-tests.mk
@@ -2,7 +2,7 @@
 ## testsuite.  This fragment is meant to be included by the Makefile.am,
 ## but also to be executed directly by make when bootstrapping automake.
 
-## Copyright (C) 2011-2020 Free Software Foundation, Inc.
+## Copyright (C) 2011-2021 Free Software Foundation, Inc.
 ##
 ## 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
@@ -848,6 +848,7 @@ t/test-trs-recover.sh \
 t/test-trs-recover2.sh \
 t/test-extensions.sh \
 t/test-extensions-cond.sh \
+t/toplevelmd.sh \
 t/parse.sh \
 t/percent.sh \
 t/percent2.sh \
diff --git a/t/toplevelmd.sh b/t/toplevelmd.sh
new file mode 100644
index 000000000..0634c7490
--- /dev/null
+++ b/t/toplevelmd.sh
@@ -0,0 +1,68 @@
+#! /bin/sh
+# Copyright (C) 2003-2021 Free Software Foundation, Inc.
+#
+# 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/>.
+
+# Check that the top-level files (INSTALL, NEWS, README-alpha, etc.)
+# can be .md, or not. (Based on alpha2.sh.)
+
+. test-init.sh
+
+cat > configure.ac << 'END'
+AC_INIT([alpha], [1.0b])
+AM_INIT_AUTOMAKE([readme-alpha])
+AC_CONFIG_FILES([Makefile sub/Makefile])
+AC_OUTPUT
+END
+
+cat > Makefile.am << 'END'
+SUBDIRS = sub
+check-local: distdir
+	for f in AUTHORS ChangeLog INSTALL NEWS README THANKS; do \
+	  test -f $(distdir)/$$f.md; done
+	test -f $(distdir)/COPYING
+	test -f $(distdir)/README-alpha.md
+	test ! -f $(distdir)/sub/README.md
+	test ! -f $(distdir)/sub/README-alpha.md # not distributed
+	: > works
+END
+
+mkdir sub
+: > sub/Makefile.am
+
+# do both md and non-md.
+: > README-alpha.md
+: > sub/README-alpha.md
+: > sub/README
+
+# top level
+: > AUTHORS.md
+: > ChangeLog.md
+: > INSTALL.md
+: > NEWS.md
+: > README.md
+: > THANKS.md
+
+# not md
+: > COPYING
+
+
+$ACLOCAL
+$AUTOCONF
+$AUTOMAKE
+./configure
+$MAKE check
+test -f works
+
+exit 33