Add a fallback test for ldns system library

Some systems don't have the pkg-config file, or don't have it in the
right place.  Breaking build on those systems does have the positive
effect of getting people to file bugs with those distros, but we
should at least keep things working on Debian wheezy while getting
Debian to accept the fix to ldns.

configure.ac

@@ -1136,7 +1136,10 @@ PKG_CHECK_MODULES([YAML], [yaml-0.1 >= 0.1.4],[
   AC_MSG_RESULT([no]); AM_CONDITIONAL([HAVE_YAML],[false])])
 PKG_CHECK_MODULES([LDNS], [libldns >= 1.6.6],[
   AM_CONDITIONAL([HAVE_LDNS],[true])],[
-  AC_MSG_RESULT([no]); AM_CONDITIONAL([HAVE_LDNS],[false])])
+  AC_CHECK_LIB([ldns], [ldns_str2rdf_a], [LDNS_LIBS=-lldns])
+  AS_IF([test -z "$LDNS_LIBS"],[
+    AC_MSG_RESULT([no]); AM_CONDITIONAL([HAVE_LDNS],[false])],[
+    AM_CONDITIONAL([HAVE_LDNS],[true])])])
 
 PKG_CHECK_MODULES([MEMCACHED], [libmemcached >= 0.31],[
   AM_CONDITIONAL([HAVE_MEMCACHED],[true])