Whoops, forgot to commit the actual fix. :)

src/render/SDL_yuv_sw.c

@@ -91,7 +91,7 @@
 /* The colorspace conversion functions */
 
 /* !!! FIXME: this broke on Clang (if it wasn't broken _before_) in https://hg.libsdl.org/SDL/rev/2ee7d2fa299b */
-#if (__GNUC__ > 2) && defined(__i386__) && __OPTIMIZE__ && SDL_ASSEMBLY_ROUTINES
+#if (__GNUC__ > 2) && defined(__i386__) && __OPTIMIZE__ && SDL_ASSEMBLY_ROUTINES && !defined(__clang__)
 #define USE_MMX_ASSEMBLY 1
 #endif