add a rule to automaticly create the common library.

Notes:
we need this idea, but this implementation doesn't work

Reference: /n/atom/patch/sorry/upascom
Date: Sat Mar  1 20:29:36 CET 2014
Signed-off-by: quanstro@quanstro.net
Reviewed-by: quanstro

--- /sys/src/cmd/upas/mkupas	Sat Mar  1 20:29:27 2014
+++ /sys/src/cmd/upas/mkupas	Sat Mar  1 20:29:28 2014
@@ -1,2 +1,5 @@
 BIN=/$objtype/bin/upas
 ABIN=/acme/bin/$objtype
+
+../common/libcommon.a$O:
+	cd ../common; mk; mk clean