clang-format.bash: update to clang-format-6.0

Update `.clang-format` with configuration to make the 6.0 format as
close as possible to what 3.8 produced before.  Then revise the style:

* Indent preprocessor directives (a feature new since 3.8)
* Add a newline and indentation before inheritance `:` and `,`

.clang-format

 ---
-# This configuration requires clang-format 3.8 or higher.
+# This configuration requires clang-format version 6.0 exactly.
 BasedOnStyle: Mozilla
 AlignOperands: false
-AlwaysBreakAfterReturnType: None
+AllowShortFunctionsOnASingleLine: InlineOnly
 AlwaysBreakAfterDefinitionReturnType: None
+AlwaysBreakAfterReturnType: None
+BinPackArguments: true
+BinPackParameters: true
+BraceWrapping:
+  AfterClass:      true
+  AfterEnum:       true
+  AfterFunction:   true
+  AfterStruct:     true
+  AfterUnion:      true
+BreakBeforeBraces: Custom
 ColumnLimit: 79
+IndentPPDirectives: AfterHash
+SortUsingDeclarations: false
+SpaceAfterTemplateKeyword: true
 Standard: Cpp03
 ...

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 **3.8** to define our style for C++ code in
+We use `clang-format`_ version **6.0** 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

@@ -78,8 +78,8 @@ test "$#" = 0 || die "$usage"
 
 # Find a default tool.
 tools='
+  clang-format-6.0
   clang-format
-  clang-format-3.8
 '
 if test "x$clang_format" = "x"; then
     for tool in $tools; do
@@ -92,7 +92,12 @@ fi
 
 # Verify that we have a tool.
 if ! type -p "$clang_format" >/dev/null; then
-    echo "Unable to locate '$clang_format'"
+    echo "Unable to locate a 'clang-format' tool."
+    exit 1
+fi
+
+if ! "$clang_format" --version | grep 'clang-format version 6\.0' >/dev/null 2>/dev/null; then
+    echo "clang-format version 6.0 is required (exactly)"
     exit 1
 fi