libmsgpack was the old C++ library provided by msgpack-c. The C library
is libmsgpackc.
The C++ support became header-only, but there was a bug
(msgpack/msgpack-c#395) wherein using msgpack-c's CMake build system
would only install libmsgpack instead of libmsgpackc.
Searching for both libraries, but preferring libmsgpackc, allows for
building against older msgpack-c releases and prepares for the upcoming
msgpack-c release which fixes the aforementioned issues.
Signed-off-by: James McCoy <jamessan@jamessan.com>