Only use memset_pattern4() if we have string.h on Apple platforms

src/stdlib/SDL_stdlib.c

@@ -641,7 +641,7 @@ SDL_memset(SDL_OUT_BYTECAP(len) void *dst, int c, size_t len)
 void *
 SDL_memset4(void *dst, Uint32 val, size_t dwords)
 {
-#ifdef __APPLE__
+#if defined(__APPLE__) && defined(HAVE_STRING_H)
     memset_pattern4(dst, &val, dwords * 4);
 #elif defined(__GNUC__) && defined(__i386__)
     int u0, u1, u2;