Remove local libgsupclient; Use libosmo-gsup-client from osmo-hlr

osmo-hlr has recently (as of Change-Id
Iad227bb477d64da30dd6bfbbe1bd0c0a55be9474) a working shared library
implementation of libosmo-gsup-client.

We can remove the local implementation in osmo-sgsn and use the
system-installed shared library instead.

Change-Id: I6f542945403cf2e3ddac419186b09ec0e2d43b69
diff --git a/configure.ac b/configure.ac
index 73b6dce..0e4ff94 100644
--- a/configure.ac
+++ b/configure.ac
@@ -46,6 +46,7 @@
 PKG_CHECK_MODULES(LIBOSMOABIS, libosmoabis >= 0.5.0)
 PKG_CHECK_MODULES(LIBOSMOGB, libosmogb >= 0.11.0)
 PKG_CHECK_MODULES(LIBOSMONETIF, libosmo-netif >= 0.2.0)
+PKG_CHECK_MODULES(LIBOSMOGSUPCLIENT, libosmo-gsup-client >= 0.2.1)
 
 # Enable/disable 3G aka IuPS + IuCS support?
 AC_ARG_ENABLE([iu], [AS_HELP_STRING([--enable-iu], [Build 3G support, aka IuPS and IuCS interfaces])],