Add DragonFlyBSD to the list of supported OSes.

patch by Adam Hoka, ahoka NetBSD org

Originally committed as revision 19190 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1632,6 +1632,8 @@ case $target_os in
     freebsd)
         disable need_memalign
         ;;
+    dragonfly)
+        ;;
     bsd/os)
         osextralibs="-lpoll -lgnugetopt"
         strip="strip -d"