libcurl.m4: updated to the latest version

m4/libcurl.m4

@@ -5,7 +5,7 @@
 #                            | (__| |_| |  _ <| |___
 #                             \___|\___/|_| \_\_____|
 #
-# Copyright (C) 2006 - 2020, David Shaw <[email protected]>
+# Copyright (C) David Shaw <[email protected]>
 #
 # This software is licensed as described in the file COPYING, which
 # you should have received as part of this distribution. The terms
@@ -18,6 +18,8 @@
 # This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 # KIND, either express or implied.
 #
+# SPDX-License-Identifier: curl
+#
 ###########################################################################
 # LIBCURL_CHECK_CONFIG ([DEFAULT-ACTION], [MINIMUM-VERSION],
 #                       [ACTION-IF-YES], [ACTION-IF-NO])