c++: add fallback mappings for C++23 and C++26

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

None of the production c++ compilers support c++26 flags yet, but this
mapping will be ready once they do.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>

mesonbuild/compilers/cpp.py

@@ -154,6 +154,10 @@ class CPPCompiler(CLikeCompiler, Compiler):
             'gnu++17': 'gnu++1z',
             'c++20': 'c++2a',
             'gnu++20': 'gnu++2a',
+            'c++23': 'c++2b',
+            'gnu++23': 'gnu++2b',
+            'c++26': 'c++2c',
+            'gnu++26': 'gnu++2c',
         }
 
         # Currently, remapping is only supported for Clang, Elbrus and GCC