build: switch out hardcoded -lresolv for ${resolv_LIBS}

Makefile.am

@@ -177,7 +177,7 @@ libgxs_authmgr_la_LIBADD = ${crypto_LIBS} ${HX_LIBS} ${jsoncpp_LIBS} libgromox_c
 EXTRA_libgxs_authmgr_la_DEPENDENCIES = ${default_sym}
 libgxs_dnsbl_filter_la_SOURCES = exch/dnsbl_filter.cpp
 libgxs_dnsbl_filter_la_LDFLAGS = ${plugin_LDFLAGS}
-libgxs_dnsbl_filter_la_LIBADD = -lresolv libgromox_common.la
+libgxs_dnsbl_filter_la_LIBADD = ${resolv_LIBS} libgromox_common.la
 EXTRA_libgxs_dnsbl_filter_la_DEPENDENCIES = ${default_sym}
 libgxs_ldap_adaptor_la_SOURCES = exch/ldap_adaptor.cpp exch/ldap_adaptor.hpp
 libgxs_ldap_adaptor_la_LDFLAGS = ${plugin_LDFLAGS}