separate libosmo-mgcp-client from mgcp server code
The mgcp code can be built with --enable-mgcp-transcoding, which adds a dependency to libgsm.
However, osmo-msc and osmo-bsc only need the mgcp client code, and including the entire libosmo-legacy-mgcp pulls in a useless requirement of libgsm.
Hence the mgcp client code that does not relate to transcoding should live in a separate library, libosmo-mgcp-client.
(osmo-bsc_nat does use the remaining bits of libosmo-legacy-mgcp, so the osmo-bsc.git in general will need to manage libgsm dependency for osmo-bsc_nat, but the osmo-bsc binary will need only the mgcp client code.)