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/src/Makefile.am b/src/Makefile.am
index f47bc00..922bbda 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -22,6 +22,7 @@
 # Libraries
 SUBDIRS = \
 	libosmo-legacy-mgcp \
+	libosmo-mgcp-client \
 	$(NULL)
 
 # Programs