building osmo-iuh and libsctp

I've recently upgraded my OS to debian 9 and suddenly have difficulty building osmo-iuh: the osmo-hnbgw linking step complains about a missing sctp_send, which is resolved by adding -lsctp to LDADD. But I'm puzzled why this comes from a system upgrade and not a code change. Does anyone have an idea what could cause this, or what would have hidden the linking error before? What's the proper way to add -lsctp? The dependency comes from libosmo-ranap using libosmo-sigtran - should the libosmo-sigtran.pc include -lsctp? - should we add a libsctp detection to osmo-iuh's configure.ac (copy from libosmo-sccp) and use $(SCTP_LIBS) constants in Makefile.am LDADD? - is everything correct and my OS has a problem instead? Thanks for any input... ~N