[readline, djgpp] Check $build_os instead of $host_os in configure

[Yao Qi]

On Wednesday, August 01, 2012 11:58:51 AM Pedro Alves wrote:
> We'd like to avoid accumulating more local readline patches.
> Can you submit this upstream first (bug-readline at gnu.org) ?
> Once it gets accepted upstream, we can apply it locally as well.

Sure.  I intended to submit this patch to readline upstream, but after looking 
for a dev mail list and source code in version control for a while on readline 
website, I am a little confused, and post patch here :)

'bug-readline at gnu.org' is the right place to go.  Thanks for your pointer.  
I'll post it there.

-- 
Yao (齐尧)