Log Message:
rtmpdump: update to 2.4.0.0.fa8646daeb19dfd12c181f7d19de708d623704c0.

This is the latest git version of the program (from 2015 though).

Switch the build to use gnutls for the command line program,
since openssl 1.1 is not supported.

Various changes since the last released version, but only git log
available. Mostly bugfixes.

Update bl3.mk file: neither zlib nor openssl nor gnutls headers/libraries
are used by the library, so remove all bl3.mk includes.