GNU: Add flags for C++26 modes

Flags added in GCC commit `5388a43f6a3` (c++: Add support for
-std={c,gnu}++2{c,6}, 2023-06-22).

Modules/Compiler/GNU.cmake

@@ -293,4 +293,9 @@ macro(__compiler_gnu_cxx_standards lang)
     set(CMAKE_${lang}20_STANDARD_COMPILE_OPTION "-std=c++2a")
     set(CMAKE_${lang}20_EXTENSION_COMPILE_OPTION "-std=gnu++2a")
   endif()
+
+  if(CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL 14.0)
+    set(CMAKE_${lang}26_STANDARD_COMPILE_OPTION "-std=c++26")
+    set(CMAKE_${lang}26_EXTENSION_COMPILE_OPTION "-std=gnu++26")
+  endif()
 endmacro()