When osmo-mgw is built --with-mgcp-transcoding, linking the mgcp library
also requires linking libgsm, even though e.g. osmo-msc never use it.
Separate the MGCP client code from the MGCP server code to avoid this dep.
The mgcp client code does use some definitions from mgcp.h and mgcp_common.c.
For simplicity, link mgcp_common.c in both libosmo-legacy-mgcp as well as
libosmo-mgcp-client. That means it is not possible to link both
libosmo-legacy-mgcp and libosmo-mgcp-client in the same binary because of
duplicate symbols, but currently that is sufficient. (An alternative would be
code dup or yet another libosmo-mgcp-common library.)
Add libosmo-mgcp-client to debian packaging.