c++: add fallback mappings for C++23

The c++23 mappings apply to current production compilers (GCC, Clang).

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Backported-by: Nirbheek Chauhan <nirbheek.chauhan@gmail.com>

mesonbuild/compilers/cpp.py

@@ -154,6 +154,8 @@ class CPPCompiler(CLikeCompiler, Compiler):
             'gnu++17': 'gnu++1z',
             'c++20': 'c++2a',
             'gnu++20': 'gnu++2a',
+            'c++23': 'c++2b',
+            'gnu++23': 'gnu++2b',
         }
 
         # Currently, remapping is only supported for Clang, Elbrus and GCC