Add AC_COPYRIGHT macro

[yadij]

To display The Squid Software Foundation copyright blurb in
./configure --version output and mentions at the top of
./configure script code.

[yadij]

Diff of ./configure --version output:

@@ -4,3 +4,10 @@
 Copyright (C) 2023 Free Software Foundation, Inc.
 This configure script is free software; the Free Software Foundation
 gives unlimited permission to copy, distribute and modify it.
+
+Copyright (C) 1996-2026 The Squid Software Foundation and contributors
+
+ Squid software is distributed under GPLv2+ license and includes
+ contributions from numerous individuals and organizations.
+ Please see the COPYING and CONTRIBUTORS files for details.
+

Diff of ./configure script:

@@ -12,6 +12,13 @@
 #
 # This configure script is free software; the Free Software Foundation
 # gives unlimited permission to copy, distribute and modify it.
+#
+# Copyright (C) 1996-2026 The Squid Software Foundation and contributors
+#
+#  Squid software is distributed under GPLv2+ license and includes
+#  contributions from numerous individuals and organizations.
+#  Please see the COPYING and CONTRIBUTORS files for details.
+#
 ## -------------------- ##
 ## M4sh Initialization. ##
 ## -------------------- ##
@@ -2225,6 +2232,13 @@
 Copyright (C) 2023 Free Software Foundation, Inc.
 This configure script is free software; the Free Software Foundation
 gives unlimited permission to copy, distribute and modify it.
+
+Copyright (C) 1996-2026 The Squid Software Foundation and contributors
+
+ Squid software is distributed under GPLv2+ license and includes
+ contributions from numerous individuals and organizations.
+ Please see the COPYING and CONTRIBUTORS files for details.
+
 _ACEOF
   exit
 fi

[squidadm]

queued for backport to v7