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.

t/hello.o.do

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