Whoops, forgot to commit the actual fix. :)
diff --git a/src/render/SDL_yuv_sw.c b/src/render/SDL_yuv_sw.c
index 737223c..facab0e 100644
--- a/src/render/SDL_yuv_sw.c
+++ b/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