add werror flags when making a debug build using llvm (#4338)

this would get bugs like
https://github.com/Cockatrice/Cockatrice/pull/4337
get signalled earlier to us

CMakeLists.txt

@@ -134,7 +134,11 @@ ELSEIF (CMAKE_COMPILER_IS_GNUCXX)
 ELSE()
     # other: osx/llvm, bsd/llvm
     set(CMAKE_CXX_FLAGS_RELEASE "-O2")
-    set(CMAKE_CXX_FLAGS_DEBUG "-g -O0")
+    if(WARNING_AS_ERROR)
+      set(CMAKE_CXX_FLAGS_DEBUG "-g -O0 -Wall -Wextra -Werror")
+    else()
+      set(CMAKE_CXX_FLAGS_DEBUG "-g -O0 -Wall -Wextra")
+    endif()
 ENDIF()
 
 # GNU systems need to define the Mersenne exponent for the RNG to compile w/o warning