Add c++23 to the list of C++ standards.

mesonbuild/compilers/cpp.py

@@ -384,11 +384,15 @@ class GnuCPPCompiler(GnuCompiler, CPPCompiler):
                 False,
             )
         })
-        opts[key].choices = [
+        cppstd_choices = [
             'none', 'c++98', 'c++03', 'c++11', 'c++14', 'c++17', 'c++1z',
             'c++2a', 'c++20', 'gnu++03', 'gnu++11', 'gnu++14', 'gnu++17',
             'gnu++1z', 'gnu++2a', 'gnu++20',
         ]
+        if version_compare(self.version, '>=12.2.0'):
+            cppstd_choices.append('c++23')
+            cppstd_choices.append('gnu++23')
+        opts[key].choices = cppstd_choices
         if self.info.is_windows() or self.info.is_cygwin():
             opts.update({
                 key.evolve('winlibs'): coredata.UserArrayOption(