PATCH, libiberty: eliminate build warning

[Nick Clifton]

Hi Ben,

> 2009-11-19  Ben Elliston  <bje@au.ibm.com>
> 
> 	* pex-unix.c (pex_child_error): Define writeerr macro to avoid
> 	unused result warnings from write(3) calls.  Undefine writeerr
> 	after all uses.

Approved - please apply.

Cheers
   Nick