configure.ac: fix AC_SUBST command for STARPU_USE_TOP

configure.ac

@@ -1275,7 +1275,7 @@ else
   build_starpu_top=no
 fi
 
-AC_SUBST(STARPU_USE_TOP, test "x$build_starpu_top" = "xyes")
+AC_SUBST(STARPU_USE_TOP, $build_starpu_top)
 AM_CONDITIONAL(BUILD_STARPU_TOP, test "x$build_starpu_top" = "xyes")
 if test "x$build_starpu_top" = "xyes"; then
 	AC_DEFINE(STARPU_USE_TOP, [1], [StarPU-Top is activated])