Don't use -mwindows with Clang on Windows

Thanks to stigmha for pointing out this doesn't work in Windows.

Makefile

@@ -580,7 +580,10 @@ ifeq ($(PLATFORM),mingw32)
   endif
 
   LIBS= -lws2_32 -lwinmm -lpsapi
-  CLIENT_LDFLAGS += -mwindows
+  # clang 3.4 doesn't support this
+  ifneq ("$(CC)", $(findstring "$(CC)", "clang" "clang++"))
+    CLIENT_LDFLAGS += -mwindows
+  endif
   CLIENT_LIBS = -lgdi32 -lole32
   RENDERER_LIBS = -lgdi32 -lole32 -lopengl32