Use the right libdir for mozjs when built via make test

'make test' sets libdir to a path in the test sandbox and it propagates
to the mozjs build if it hasn't been built yet. Reinstate a sensible
libdir for mozjs.

Makefile.common

@@ -89,7 +89,7 @@ all: build
 -include $(CODE:.c=.d)
 
 $(JSOBJ):
-	$(MAKE) -C $(SRCROOT)/mozjs
+	$(MAKE) -C $(SRCROOT)/mozjs libdir=/usr/lib
 
 .PHONY: build install clean test all testclean