use correct compiler macro for DragonFlyBSD

Although i haven't tested this, the following two search results make it pretty clear it's right:
    http://gitweb.dragonflybsd.org/dragonfly.git?a=search&s=__DragonFly__
    http://gitweb.dragonflybsd.org/dragonfly.git?a=search&s=__DragonFlyBSD__