swanctl: Fix man page build on FreeBSD

BSD make seems to only evaluate $< for certain rules (like the suffix rule
used to generate the config template).

src/swanctl/Makefile.am

@@ -46,7 +46,7 @@ CLEANFILES = $(man_MANS)
 
 swanctl.conf.5.main: swanctl.opt
 	$(AM_V_GEN) \
-	$(PYTHON) $(top_srcdir)/conf/format-options.py -n -f man $< > $(srcdir)/$@
+	$(PYTHON) $(top_srcdir)/conf/format-options.py -n -f man swanctl.opt > $(srcdir)/$@
 
 swanctl.conf.5: swanctl.conf.5.head swanctl.conf.5.main swanctl.conf.5.tail
 	$(AM_V_GEN) \