Use system ldns library

This library is used only by mod_enum and is available on most
platforms.  The version in our tree has many code quality problems
that are generating noise when doing static analysis.  Suffice it to
say we want rid of it as our responsibility.

FS-353

configure.ac

@@ -1121,6 +1121,11 @@ case $host in
        ;;
 esac
 
+# temporary workaround for Debian libldns-dev package bug
+if test -f /usr/lib/pkg-config/libldns.pc; then
+  path_push_unique PKG_CONFIG_PATH /usr/lib/pkg-config
+fi
+
 PKG_CHECK_MODULES([SQLITE], [sqlite3 >= 3.6.20])
 PKG_CHECK_MODULES([CURL], [libcurl >= 7.19])
 PKG_CHECK_MODULES([PCRE], [libpcre >= 7.8])
@@ -1128,6 +1133,9 @@ PKG_CHECK_MODULES([SPEEX], [speex >= 1.2rc1 speexdsp >= 1.2rc1])
 PKG_CHECK_MODULES([YAML], [yaml-0.1 >= 0.1.4],[
   AM_CONDITIONAL([HAVE_YAML],[true])],[
   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])])
 
 PKG_CHECK_MODULES([MEMCACHED], [libmemcached >= 0.31],[
   AM_CONDITIONAL([HAVE_MEMCACHED],[true])