Combine rules for manpages.

Reviewed-by: iustinp

man/Makefile.am

 # Build man pages
-#
 
 man_MANS = ganeti.7 ganeti-os-interface.7 gnt-cluster.8 gnt-node.8 gnt-os.8 \
 	   gnt-instance.8 ganeti-noded.8 ganeti-watcher.8 gnt-backup.8
@@ -9,10 +8,6 @@ EXTRA_DIST = ganeti-os-interface.sgml gnt-cluster.sgml gnt-node.sgml \
 	     gnt-backup.sgml ganeti-noded.sgml \
 	     footer.sgml $(man_MANS)
 
-%.8: %.sgml footer.sgml
-	docbook2man $<
-	rm -f manpage.links manpage.refs
-
-%.7: %.sgml footer.sgml
+%.7 %.8: %.sgml footer.sgml
 	docbook2man $<
 	rm -f manpage.links manpage.refs