Features: Activate C++ 20 support for GNU 8.0+

Modules/Compiler/GNU-CXX.cmake

@@ -41,4 +41,9 @@ elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
   set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
 endif()
 
+if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 8.0)
+  set(CMAKE_CXX20_STANDARD_COMPILE_OPTION "-std=c++2a")
+  set(CMAKE_CXX20_EXTENSION_COMPILE_OPTION "-std=gnu++2a")
+endif()
+
 __compiler_check_default_language_standard(CXX 3.4 98 6.0 14)