Applied DragonFly BSD patch

https://raw.githubusercontent.com/DragonFlyBSD/DeltaPorts/master/ports/devel/sdl20/dragonfly/patch-include_SDL__endian.h

include/SDL_endian.h

@@ -59,7 +59,7 @@ _m_prefetch(void *__P)
 #ifdef __linux__
 #include <endian.h>
 #define SDL_BYTEORDER  __BYTE_ORDER
-#elif defined(__OpenBSD__)
+#elif defined(__OpenBSD__) || defined(__DragonFly__)
 #include <endian.h>
 #define SDL_BYTEORDER  BYTE_ORDER
 #elif defined(__FreeBSD__) || defined(__NetBSD__)