Merge FreeBSD and DragonFlyBSD OS detection sections.

Apparently DragonFlyBSD does not need memalign either.

Originally committed as revision 19191 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1629,11 +1629,9 @@ case $target_os in
         oss_demuxer_extralibs="-lossaudio"
         oss_muxer_extralibs="-lossaudio"
         ;;
-    freebsd)
+    freebsd|dragonfly)
         disable need_memalign
         ;;
-    dragonfly)
-        ;;
     bsd/os)
         osextralibs="-lpoll -lgnugetopt"
         strip="strip -d"