Meta: Remove obsolete `-fconcepts` CFLAG

The GCC documentation says that since it's officially a part of C++20,
this flag does nothing. Clang, however, does complain that it does not
recognize it, so it's better to just remove it.

CMakeLists.txt

@@ -101,7 +101,7 @@ set(CMAKE_CXX_EXTENSIONS OFF)
 
 add_compile_options(-Wno-literal-suffix)
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
-    add_compile_options(-fconcepts)
+    add_compile_options(-Wno-literal-suffix)
 elseif ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang$")
     add_compile_options(-Wno-overloaded-virtual -Wno-user-defined-literals)
 endif()