D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/tools/diffutils/source/m4/error.m4

@@ -0,0 +1,27 @@
+#serial 14
+
+# Copyright (C) 1996-1998, 2001-2004, 2009-2017 Free Software Foundation, Inc.
+#
+# This file is free software; the Free Software Foundation
+# gives unlimited permission to copy and/or distribute it,
+# with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_ERROR],
+[
+  dnl We don't use AC_FUNC_ERROR_AT_LINE any more, because it is no longer
+  dnl maintained in Autoconf and because it invokes AC_LIBOBJ.
+  AC_CACHE_CHECK([for error_at_line], [ac_cv_lib_error_at_line],
+    [AC_LINK_IFELSE(
+       [AC_LANG_PROGRAM(
+          [[#include <error.h>]],
+          [[error_at_line (0, 0, "", 0, "an error occurred");]])],
+       [ac_cv_lib_error_at_line=yes],
+       [ac_cv_lib_error_at_line=no])])
+])
+
+# Prerequisites of lib/error.c.
+AC_DEFUN([gl_PREREQ_ERROR],
+[
+  AC_REQUIRE([AC_FUNC_STRERROR_R])
+  :
+])