https://bugs.gentoo.org/563922
http://bugs.ntp.org/show_bug.cgi?id=2906

--- a/sntp/tests/Makefile.in
+++ b/sntp/tests/Makefile.in
@@ -783,6 +783,7 @@ base_LDADD = \
 	$(LIBOPTS_LDADD)			\
 	$(LDADD_LIBEVENT)			\
 	$(top_builddir)/../libntp/libntp.a	\
+	$(PTHREAD_LIBS) \
 	$(LDADD_LIBNTP)				\
 	$(LDADD_NTP)				\
 	$(NULL)