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/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index 6771b31..1b45014 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -29,6 +29,8 @@
 
 #include <osmocore/talloc.h>
 
+#include <sccp/sccp.h>
+
 #include <stdio.h>
 
 /* test messages for ipa */
@@ -699,6 +701,7 @@
 {
 	struct log_target *stderr_target;
 
+	sccp_set_log_area(DSCCP);
 	log_init(&log_info);
 	stderr_target = log_target_create_stderr();
 	log_add_target(stderr_target);