Avoid Clang c++11-narrowing error

Downgrades c++11-narrowing to a warning when compiling with Clang. A
better solution would be to fix the error, but this at least brings
Clang in line with the more permissive GCC, which treats this as a
warning by default.

CMakeLists.txt

@@ -21,6 +21,12 @@ if("${CMAKE_INTERPROCEDURAL_OPTIMIZATION}" STREQUAL "")
 	endif()
 endif()
 
+# FIXME: Clang treats this as an error by default; fixing the source rather
+# than downgrading the error to a warning would be a better solution.
+add_compile_options(
+	$<$<COMPILE_LANG_AND_ID:CXX,AppleClang,Clang>:-Wno-error=c++11-narrowing>
+	)
+
 find_package(SDL2 REQUIRED)
 
 if(PLATFORM STREQUAL "MAC")