Add DragonFlyBSD to the list of supported OSes.

patch by Adam Hoka, ahoka NetBSD org


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19190 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

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