Add support for --with-system-gdbinit

[Jerome Guitton]

Eli Zaretskii (eliz@gnu.org):

> > Date: Fri, 23 Jan 2009 18:24:46 +0100
> > From: Jerome Guitton <guitton@adacore.com>
> > Cc: gdb-patches@sourceware.org
> > 
> > > This part is approved, with a few minor comments
> > 
> > Thank you for the review Eli. I have attached the new patch; I will commit
> > it when the other part is approved.
> 
> Thanks, this version is OK.

This has been committed as well. Thanks!