cosmetic: Add note about libosmo-legacy-mgcp to configure.ac

Change-Id: Ia496bd9ec02c0028aa541a798b1358a68ad18e21
Closes: OS#3226
diff --git a/configure.ac b/configure.ac
index c74402a..a077fd5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -48,6 +48,9 @@
 PKG_CHECK_MODULES(LIBOSMOSIGTRAN, libosmo-sigtran >= 0.8.0)
 PKG_CHECK_MODULES(LIBOSMOSCCP, libosmo-sccp >= 0.0.2)
 PKG_CHECK_MODULES(LIBOSMOMGCPCLIENT, libosmo-mgcp-client >= 1.2.0)
+
+#NOTE: osmo-bsc does not depend on libosmo-legacy-mgcp anymore, but we still
+#      need the dependancy for osmo-bsc-nat, which still uses the old API.
 PKG_CHECK_MODULES(LIBOSMOLEGACYMGCP, libosmo-legacy-mgcp >= 1.0.0)
 
 dnl checks for header files