redo-ifcreate: barf if the file already exists.

I'm pretty sure this must be the intended behaviour.  It's kind of
meaningless to use this to declare a dependency on a file that might start
to exist later, if the file already exists.

redo-ifcreate.py

@@ -11,6 +11,7 @@ if not vars.TARGET:
 for t in sys.argv[1:]:
     mkdirp('%s/.redo' % vars.BASE)
     if os.path.exists(t):
-        add_dep(vars.TARGET, 'm', t)
+        err('redo-ifcreate: error: %r already exists\n' % t)
+        sys.exit(1)
     else:
         add_dep(vars.TARGET, 'c', t)

t/hello.o.do

@@ -1,2 +1,3 @@
 redo-ifchange CC hello.c /usr/include/stdio.h
+redo-ifcreate stdio.h
 ./CC hello.c