swanctl: Explicitly link against -lpthread and -ldl if required

We already do this for charon, as some toolchains require an explicit
link even if libstrongswan already depends on it.

src/swanctl/Makefile.am

@@ -25,7 +25,8 @@ swanctl_SOURCES = \
 
 swanctl_LDADD = \
 	$(top_builddir)/src/libcharon/plugins/vici/libvici.la \
-	$(top_builddir)/src/libstrongswan/libstrongswan.la
+	$(top_builddir)/src/libstrongswan/libstrongswan.la \
+	$(PTHREADLIB) $(DLLIB)
 
 swanctl.o :		$(top_builddir)/config.status