fnmatch: Ensure that on Cygwin ≥ 3.5.0, Cygwin's native fnmatch is used

[Bruno Haible]

In the forthcoming Cygwin 3.5.0, the fnmatch() function is greatly
improved, implementing not only character classes and support for all
Unicode characters (like Gnulib's fnmatch() now has), but also equivalence
classes and collating symbols. So, in this situation, programs that
don't request the module 'fnmatch-gnu' should use Cygwin's native fnmatch().


2023-07-28  Bruno Haible  <bruno@clisp.org>

        fnmatch: Ensure that on Cygwin ≥ 3.5.0, Cygwin's native fnmatch is used.
        Tested by Corinna Vinschen in
        <https://cygwin.com/pipermail/cygwin/2023-July/254036.html>.
        * m4/fnmatch.m4 (gl_FUNC_FNMATCH_POSIX): On Cygwin, don't force
        REPLACE_FNMATCH to 1 just because wchar_t is small.

diff --git a/m4/fnmatch.m4 b/m4/fnmatch.m4
index 2e1442eff7..e99737a476 100644
--- a/m4/fnmatch.m4
+++ b/m4/fnmatch.m4
@@ -1,4 +1,4 @@
-# Check for fnmatch - serial 18  -*- coding: utf-8 -*-
+# Check for fnmatch - serial 19  -*- coding: utf-8 -*-
 
 # Copyright (C) 2000-2007, 2009-2023 Free Software Foundation, Inc.
 # This file is free software; the Free Software Foundation
@@ -14,7 +14,7 @@ AC_DEFUN([gl_FUNC_FNMATCH_POSIX]
   m4_divert_text([DEFAULTS], [gl_fnmatch_required=POSIX])
 
   AC_REQUIRE([gl_FNMATCH_H])
-  AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
+  AC_REQUIRE([AC_CANONICAL_HOST])
   gl_fnmatch_required_lowercase=`
     echo $gl_fnmatch_required | LC_ALL=C tr '[[A-Z]]' '[[a-z]]'
   `
@@ -164,7 +164,17 @@ AC_DEFUN([gl_FUNC_FNMATCH_POSIX]
     dnl This is due to wchar_t being only 16 bits wide.
     AC_REQUIRE([gl_UCHAR_H])
     if test $SMALL_WCHAR_T = 1; then
-      REPLACE_FNMATCH=1
+      case "$host_os" in
+        cygwin*)
+          dnl On Cygwin < 3.5.0, the above $gl_fnmatch_result came out as 'no',
+          dnl On Cygwin >= 3.5.0, fnmatch supports all Unicode characters,
+          dnl despite wchar_t being only 16 bits wide (because internally it
+          dnl works on wint_t values).
+          ;;
+        *)
+          REPLACE_FNMATCH=1
+          ;;
+      esac
     fi
   fi
   if test $HAVE_FNMATCH = 0 || test $REPLACE_FNMATCH = 1; then