Tweak for no-libio-install

[Chip Salzenberg]

According to Benjamin Kosnik:
> Why is it not working for you, or am I not understanding the problem?

At bottom, it's just a shell syntax error when $(libio_headers) is empty.

Consider this statement from a Makefile:
    for f in $(MACRO); do something; done
When $(MACRO) is empty, this expands to:
    for f in ; do something; done
This is a Bourne shell syntax error.  (Or at least bash thinks so.)
The phrase "for f in" must be followed by a non-empty list.

All my patch was intended to do is eliminate that error by commenting
out the problematic loop when its macro is empty.

How does the patch break cross-compilation?  It seems harmless to me.
-- 
Chip Salzenberg              - a.k.a. -              <chip@valinux.com>
"I wanted to play hopscotch with the impenetrable mystery of existence,
    but he stepped in a wormhole and had to go in early."  // MST3K