* fix bogus GPL-3+ text
* update dates and meta info
* complete list of contributors

belle-sip now uses belr to generate the SDP parser at run-time.
Because BelleSIPTargets-none.cmake expresses "bctoolbox;belr" as
the libraries to link I think libbellesip-dev needs to express a
corresponding dependency on libbelr-dev, too, but I could be wrong.
Ibidem for belle-sip.pc.in.

Update to upstream version '4.4.21+dfsg'
with Debian dir cfe21fd490aae6911999ec58d95e815ee3c62314

Closes: #977556

Upstream is using cmake but has autoconf lying around