clang-format.bash: update to clang-format-18

The `.clang-format` configuration needs no changes to make the
version 18 format close to what version 15 produced before.

.clang-format

 ---
-# This configuration requires clang-format version 15 exactly.
+# This configuration requires clang-format version 18 exactly.
 BasedOnStyle: Mozilla
 AlignOperands: false
 AllowShortFunctionsOnASingleLine: InlineOnly

.gitattributes

@@ -4,7 +4,7 @@
 .hooks*          export-ignore
 
 # Custom attribute to mark sources as using the kwsys C code style.
-[attr]kwsys-c-style  whitespace=tab-in-indent  format.clang-format=15
+[attr]kwsys-c-style  whitespace=tab-in-indent  format.clang-format=18
 
 /GitSetup        export-ignore
 /*.sh            export-ignore eol=lf

CONTRIBUTING.rst

@@ -27,7 +27,7 @@ copies of KWSys within dependent projects can be updated to get the changes.
 Code Style
 ==========
 
-We use `clang-format`_ version **15** to define our style for C++ code in
+We use `clang-format`_ version **18** to define our style for C++ code in
 the KWSys source tree.  See the `.clang-format`_ configuration file for
 our style settings.  Use the `clang-format.bash`_ script to format source
 code.  It automatically runs ``clang-format`` on the set of source files

clang-format.bash

 #!/usr/bin/env bash
 #=============================================================================
-# Copyright 2015-2017 Kitware, Inc.
+# Copyright 2015-2025 Kitware, Inc.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -78,7 +78,7 @@ test "$#" = 0 || die "$usage"
 
 # Find a default tool.
 tools='
-  clang-format-15
+  clang-format-18
   clang-format
 '
 if test "x$clang_format" = "x"; then
@@ -96,8 +96,8 @@ if ! type -p "$clang_format" >/dev/null; then
     exit 1
 fi
 
-if ! "$clang_format" --version | grep 'clang-format version 15' >/dev/null 2>/dev/null; then
-    echo "clang-format version 15 is required (exactly)"
+if ! "$clang_format" --version | grep 'clang-format version 18' >/dev/null 2>/dev/null; then
+    echo "clang-format version 18 is required (exactly)"
     exit 1
 fi
 
@@ -117,7 +117,7 @@ list_files() {
 
   # Select sources with our attribute.
   git check-attr --stdin format.clang-format |
-    sed -n '/: format\.clang-format: \(set\|15\)$/ {s/:[^:]*:[^:]*$//p}'
+    sed -n '/: format\.clang-format: \(set\|18\)$/ {s/:[^:]*:[^:]*$//p}'
 }
 
 # Transform configured sources to protect @SYMBOLS@.