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/Makefile.am b/openbsc/tests/Makefile.am
index b469832..99ec296 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -1 +1,5 @@
-SUBDIRS = debug gsm0408 db channel sccp bsc-nat
+SUBDIRS = debug gsm0408 db channel
+
+if BUILD_NAT
+SUBDIRS += bsc-nat
+endif