separate libosmo-mgcp-client from mgcp server code

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.

Related: OS#2488
Change-Id: I8e0b2d2a399b77086a36606f5e427271c6242df1
diff --git a/tests/Makefile.am b/tests/Makefile.am
index f6cb938..ae51f89 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,5 +1,6 @@
 SUBDIRS = \
 	legacy_mgcp \
+	mgcp_client \
 	$(NULL)
 
 # The `:;' works around a Bash 3.2 bug when the output is not writeable.