Build: Make things build with clang without needing local changes

Useful for sanitizer fuzzer builds.

clang doesn't have a -fconcepts switch (I'm guessing it just enables
concepts automatically with -std=c++2a, but I haven't checked),
and at least the version on my system doesn't understand
-Wno-deprecated-move, so pass these two flags only to gcc.
In return, disable -Woverloaded-virtual which fires in many places.

The preceding commits fixed the handful of -Wunused-private-field
warnings that clang emitted.

CMakeLists.txt

@@ -40,7 +40,12 @@ add_custom_target(check-style
  USES_TERMINAL
 )
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -std=c++2a -fdiagnostics-color=always -fconcepts")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -std=c++2a -fdiagnostics-color=always")
+if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fconcepts")
+elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-overloaded-virtual")
+endif()
 
 include_directories(Libraries)
 include_directories(.)

Meta/Lagom/CMakeLists.txt

@@ -1,10 +1,13 @@
 cmake_minimum_required (VERSION 3.0)
 project (Lagom)
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -Wall -Wextra -Werror -std=c++2a -fPIC -g -Wno-deprecated-copy")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -Wall -Wextra -Werror -std=c++2a -fPIC -g")
+if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-deprecated-copy")
+endif()
 
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
- set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wconsumed")
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wconsumed -Wno-overloaded-virtual")
 
  option(ENABLE_ADDRESS_SANITIZER "Enable address sanitizer testing in gcc/clang" FALSE)
  if (ENABLE_ADDRESS_SANITIZER)