Makefile.am: use the existing rule to build starpu-top

Makefile.am

@@ -68,8 +68,7 @@ nodist_versinclude_HEADERS = 			\
 	include/starpu_config.h
 
 if BUILD_STARPU_TOP
-starpu-top/starpu_top$(EXEEXT):
-	cd starpu-top ; $(QMAKE) ; $(MAKE)
+starpu-top/starpu_top$(EXEEXT): all-local
 all-local:
 	cd starpu-top ; $(QMAKE) ; $(MAKE)
 clean-local: