Don't build unnecessary SDL sources on OSX

These files don't compile with Xcode 4.4+ due to relying on
long-deprecated and now removed Carbon headers. Turns out the code in
them isn't actually used on OSX, and the upstream build system doesn't
try to build them.

Change-Id: I1e3dcdd9e7e598ede05a0a2ee038c16a195ebdab
diff --git a/distrib/sdl-1.2.15/sources.make b/distrib/sdl-1.2.15/sources.make
index 5117506..361fcfa 100644
--- a/distrib/sdl-1.2.15/sources.make
+++ b/distrib/sdl-1.2.15/sources.make
@@ -183,12 +183,7 @@
           quartz/SDL_QuartzVideo.m \
           quartz/SDL_QuartzWM.m \
           quartz/SDL_QuartzWindow.m \
-          quartz/SDL_QuartzEvents.m \
-          maccommon/SDL_macevents.c \
-          maccommon/SDL_macgl.c \
-          maccommon/SDL_macmouse.c \
-          maccommon/SDL_macwm.c
-
+          quartz/SDL_QuartzEvents.m
 endif
 
 ifeq ($(SDL_CONFIG_VIDEO_X11),yes)