sccp: Use the external libosmo-sccp as sccp implementation

Add --enable-nat and --enable-osmo-bsc to build applications
requiring the Osmo SCCP library to be installed. We are not
using autodiscover as this is out of fashion.
diff --git a/openbsc/tests/bsc-nat/Makefile.am b/openbsc/tests/bsc-nat/Makefile.am
index 013a465..f5af721 100644
--- a/openbsc/tests/bsc-nat/Makefile.am
+++ b/openbsc/tests/bsc-nat/Makefile.am
@@ -1,5 +1,5 @@
 INCLUDES = $(all_includes) -I$(top_srcdir)/include
-AM_CFLAGS=-Wall -ggdb3 $(LIBOSMOCORE_CFLAGS)
+AM_CFLAGS=-Wall -ggdb3 $(LIBOSMOCORE_CFLAGS) $(LIBOSMOSCCP_CFLAGS)
 
 EXTRA_DIST = bsc_data.c
 
@@ -12,4 +12,4 @@
 			$(top_srcdir)/src/nat/bsc_mgcp_utils.c \
 			$(top_srcdir)/src/mgcp/mgcp_protocol.c \
 			$(top_srcdir)/src/mgcp/mgcp_network.c
-bsc_nat_test_LDADD = $(top_builddir)/src/libbsc.a $(top_builddir)/src/libsccp.a $(LIBOSMOCORE_LIBS) -lrt
+bsc_nat_test_LDADD = $(top_builddir)/src/libbsc.a $(LIBOSMOCORE_LIBS) -lrt $(LIBOSMOSCCP_LIBS)