Fix the end-to-end make process.

Previously, when doing something like "make clean test", this would
check for the existence of sys.ji before doing any work, find that
it exists, decide to use it, then delete it and fail when trying to
use it later.

Makefile

@@ -16,12 +16,8 @@ sys0.ji: src/boot.j src/dump.c j/stage0.j
 	$(QUIET_JULIA) ./julia -b stage0.j
 
 # if sys.ji exists, use it to rebuild, otherwise use sys0.ji
-sys.ji: sys0.ji VERSION j/sysimg.j j/start_image.j j/*.j
-ifeq ($(wildcard sys.ji),)
-	$(QUIET_JULIA) ./julia -J sys0.ji stage1.j
-else
-	$(QUIET_JULIA) ./julia stage1.j
-endif
+sys.ji: VERSION sys0.ji j/stage1.j j/sysimg.j j/start_image.j j/*.j
+	$(QUIET_JULIA) ./julia `test -f sys.ji && echo stage1.j || echo -J sys0.ji stage1.j`
 
 install: release
 	install -d $(DESTDIR)/usr/share/julia/lib