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/strings_h.m4

@@ -0,0 +1,52 @@
+# Configure a replacement for <strings.h>.
+# serial 6
+
+# Copyright (C) 2007, 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_HEADER_STRINGS_H],
+[
+  dnl Use AC_REQUIRE here, so that the default behavior below is expanded
+  dnl once only, before all statements that occur in other macros.
+  AC_REQUIRE([gl_HEADER_STRINGS_H_BODY])
+])
+
+AC_DEFUN([gl_HEADER_STRINGS_H_BODY],
+[
+  AC_REQUIRE([gl_HEADER_STRINGS_H_DEFAULTS])
+
+  gl_CHECK_NEXT_HEADERS([strings.h])
+  if test $ac_cv_header_strings_h = yes; then
+    HAVE_STRINGS_H=1
+  else
+    HAVE_STRINGS_H=0
+  fi
+  AC_SUBST([HAVE_STRINGS_H])
+
+  dnl Check for declarations of anything we want to poison if the
+  dnl corresponding gnulib module is not in use.
+  gl_WARN_ON_USE_PREPARE([[
+    /* Minix 3.1.8 has a bug: <sys/types.h> must be included before
+       <strings.h>.  */
+    #include <sys/types.h>
+    #include <strings.h>
+    ]], [ffs strcasecmp strncasecmp])
+])
+
+AC_DEFUN([gl_STRINGS_MODULE_INDICATOR],
+[
+  dnl Use AC_REQUIRE here, so that the default settings are expanded once only.
+  AC_REQUIRE([gl_HEADER_STRINGS_H_DEFAULTS])
+  gl_MODULE_INDICATOR_SET_VARIABLE([$1])
+])
+
+AC_DEFUN([gl_HEADER_STRINGS_H_DEFAULTS],
+[
+  GNULIB_FFS=0;            AC_SUBST([GNULIB_FFS])
+  dnl Assume proper GNU behavior unless another module says otherwise.
+  HAVE_FFS=1;              AC_SUBST([HAVE_FFS])
+  HAVE_STRCASECMP=1;       AC_SUBST([HAVE_STRCASECMP])
+  HAVE_DECL_STRNCASECMP=1; AC_SUBST([HAVE_DECL_STRNCASECMP])
+])