Turn libosmo-mgcp into local, non-installed library

This is an internal library simmilar to 'libmsc' in osmo-msc, which
we don't expect to be used by other programs except osmo-mgw.  Hence,
there's no need to install it as a shared library, which introduces
requirements about ABI/API stability and the like.

osmo-bsc_nat uses libosmo-legacy-mgcp, and once we should rewrite
osmo-bsc_nat, we might need some of the libosmo-mgcp related functions,
but at this point it's unclear what exactly would be needed and if
current libosmo-mgcp can provide that.  As needed, we can introduce
a related shared library at that point.

Change-Id: Iba0a2c9c694e360356ac2ca584e97795281c6198
9 files changed