Add clang-format config file for formatting C++ code

.clang-format

+# This file should be kept in sync with the one in dynare.git (which also
+# contains more explanations).
+Language: Cpp
+Standard: c++20
+ColumnLimit: 100
+BasedOnStyle: GNU
+AllowShortFunctionsOnASingleLine: InlineOnly
+AlwaysBreakTemplateDeclarations: Yes
+BreakConstructorInitializers: AfterColon
+BreakInheritanceList: AfterColon
+Cpp11BracedListStyle: true
+DeriveLineEnding: false
+IndentPPDirectives: AfterHash
+PPIndentWidth: 1
+PointerAlignment: Left
+SpaceAfterTemplateKeyword: false
+SpaceBeforeParens: ControlStatements
+SpaceBeforeCpp11BracedList: true

meson.build

@@ -3,6 +3,7 @@
 
 project('dynare-preprocessor', 'cpp',
         version : '6-unstable',
+        # NB: update C++ standard in .clang-format whenever the following is modified
         default_options : [ 'cpp_std=gnu++20', 'warning_level=2' ],
         meson_version : '>=0.64.0')