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/libosmo-mgcp-client.pc.in b/libosmo-mgcp-client.pc.in
new file mode 100644
index 0000000..aee86dc
--- /dev/null
+++ b/libosmo-mgcp-client.pc.in
@@ -0,0 +1,10 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: Osmocom Media Gateway Control Protocol Client library
+Description: C Utility Library
+Version: @VERSION@
+Libs: -L${libdir} -losmo-mgcp-client
+Cflags: -I${includedir}/