Build: Use -fconcepts flag to enable C++20 concepts.

Author: https://github.com/emanuele6
Commit: 75a4b1a27e
Pull-request: https://github.com/SerenityOS/serenity/pull/2937
Reviewed-by: https://github.com/awesomekling

CMakeLists.txt

@@ -40,7 +40,7 @@ add_custom_target(check-style
     USES_TERMINAL
 )
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -std=c++2a -fdiagnostics-color=always")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -std=c++2a -fdiagnostics-color=always -fconcepts")
 
 include_directories(Libraries)
 include_directories(.)