osmo-hnbgw lacks support for an co-located media gateway. This makes it
virtually impossible to isolate the HNB from the core network properly.
Lets add MGCP support to osmo-hnbgw so that it can control a co-located
media gateway to relay the RTP streams between HNB and core network.
We build with --enable-werror during development and in CI. If the code
is built with a different compiler that throws additional warnings, it
should not stop the build.
This patch also effectively removes dependency on autoconf-archive.