Export enabled library dependencies for static pkg-config consumers.

--- configure.ac.orig	2026-10-13 11:57:24 UTC
+++ configure.ac
@@ -328,6 +328,15 @@
     ])
 ])
 AM_CONDITIONAL([HAVE_LIBXML2], [test "x$LIBXML2_LIBS" != "x"])
+
+LIBNBD_REQUIRES_PRIVATE=
+AS_IF([test "x$GNUTLS_LIBS" != "x"],[
+    LIBNBD_REQUIRES_PRIVATE="gnutls"
+])
+AS_IF([test "x$LIBXML2_LIBS" != "x"],[
+    LIBNBD_REQUIRES_PRIVATE="$LIBNBD_REQUIRES_PRIVATE libxml-2.0"
+])
+AC_SUBST([LIBNBD_REQUIRES_PRIVATE])
 
 dnl glib2 main loop for examples that interoperate with the glib main loop.
 PKG_CHECK_MODULES([GLIB], [glib-2.0], [