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/localtime-buffer.m4

@@ -0,0 +1,21 @@
+# localtime-buffer.m4 serial 1
+dnl Copyright (C) 2017 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_LOCALTIME_BUFFER_DEFAULTS],
+[
+  NEED_LOCALTIME_BUFFER=0
+])
+
+dnl Macro invoked from other modules, to signal that the compilation of
+dnl module 'localtime-buffer' is needed.
+AC_DEFUN([gl_LOCALTIME_BUFFER_NEEDED],
+[
+  AC_REQUIRE([gl_LOCALTIME_BUFFER_DEFAULTS])
+  AC_REQUIRE([gl_HEADER_TIME_H_DEFAULTS])
+  NEED_LOCALTIME_BUFFER=1
+  REPLACE_GMTIME=1
+  REPLACE_LOCALTIME=1
+])