rtmpdump: 2.4

Update rtmpdump from 2.3 to 2.4.

Using tar.gz from https://github.com/svnpenn/rtmpdump/tags, as http://rtmpdump.mplayerhq.hu/download/ does not have 2.4. If this is dubious, could ask them to put 2.4 to the official site.

2.4 also makes the Makefile munging unnecessary, since SYS=darwin is stock-supported.