Fix finding msgpack 6+ tmate FTBFS with the 6.x version of the msgpack C library. Upstream renamed the library from libmsgpackc to libmsgpack-c and the pkg-config file from msgpack.pc to msgpack-c.pc, so any checks for the presence of the library at build time should handle both names.
Carlo Cabrera <30379873+carlocab@users.noreply.github.com>
