Fixed using libunwind on iOS, where it's only available on the simulator (thanks Vit!)

include/SDL_config_iphoneos.h

@@ -44,7 +44,8 @@
 #define HAVE_STDIO_H    1
 #define HAVE_STRING_H   1
 #define HAVE_SYS_TYPES_H    1
-#define HAVE_LIBUNWIND_H    1
+/* The libunwind functions are only available on x86 */
+/* #undef HAVE_LIBUNWIND_H */
 
 /* C library functions */
 #define HAVE_MALLOC 1