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/src/nat/Makefile.am b/openbsc/src/nat/Makefile.am
index 0ca3bdf..2f37f5c 100644
--- a/openbsc/src/nat/Makefile.am
+++ b/openbsc/src/nat/Makefile.am
@@ -1,5 +1,5 @@
 INCLUDES = $(all_includes) -I$(top_srcdir)/include -I$(top_builddir)
-AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS)
+AM_CFLAGS=-Wall $(LIBOSMOCORE_CFLAGS) $(LIBOSMOVTY_CFLAGS) $(LIBOSMOSCCP_CFLAGS)
 AM_LDFLAGS = $(LIBOSMOCORE_LIBS) $(LIBOSMOVTY_LIBS)
 
 bin_PROGRAMS = bsc_nat
@@ -8,6 +8,6 @@
 bsc_nat_SOURCES = bsc_filter.c bsc_mgcp_utils.c bsc_nat.c bsc_nat_utils.c \
 		  bsc_nat_vty.c bsc_sccp.c \
 		$(top_srcdir)/src/debug.c $(top_srcdir)/src/bsc_msc.c
-bsc_nat_LDADD = $(top_builddir)/src/libvty.a $(top_builddir)/src/libsccp.a \
+bsc_nat_LDADD = $(top_builddir)/src/libvty.a \
 		$(top_builddir)/src/libmgcp.a $(top_builddir)/src/libbsc.a \
-		-lrt
+		-lrt $(LIBOSMOSCCP_LIBS)