add mDNS lookup method to libosmo-mslookup

Add the first actually useful lookup method to the mslookup library: multicast
DNS.

The server side is added in a subsequent commit, when the mslookup server is
implemented for the osmo-hlr program.

Use custom DNS encoding instead of libc-ares (which we use in OsmoSGSN
already), because libc-ares is only a DNS client implementation and we will
need both client and server.

Related: OS#4237
Patch-by: osmith, nhofmeyr
Change-Id: I03a0ffa1d4dc1b24ac78a5ad0975bca90a49c728
diff --git a/configure.ac b/configure.ac
index 1afb5c7..ab01003 100644
--- a/configure.ac
+++ b/configure.ac
@@ -178,6 +178,7 @@
 	include/Makefile
 	include/osmocom/Makefile
 	include/osmocom/hlr/Makefile
+	include/osmocom/mslookup/Makefile
 	libosmo-gsup-client.pc
 	libosmo-mslookup.pc
 	sql/Makefile