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/configure.ac b/configure.ac
index 5482b8c..86c3bc5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -118,15 +118,19 @@
 
 AC_OUTPUT(
     libosmo-legacy-mgcp.pc
+    libosmo-mgcp-client.pc
     include/Makefile
     include/osmocom/Makefile
     include/osmocom/legacy_mgcp/Makefile
+    include/osmocom/mgcp_client/Makefile
     src/Makefile
     src/libosmo-legacy-mgcp/Makefile
+    src/libosmo-mgcp-client/Makefile
     src/osmo-bsc_mgcp/Makefile
     tests/Makefile
     tests/atlocal
     tests/legacy_mgcp/Makefile
+    tests/mgcp_client/Makefile
     doc/Makefile
     doc/examples/Makefile
     contrib/Makefile