Add support for --with-system-gdbinit

[Eli Zaretskii]

> Date: Mon, 19 Jan 2009 15:10:53 +0100
> From: Jerome Guitton <guitton@adacore.com>
> 
> At startup, GDB reads the following init files and executes their commands:
>   * system-wide init file: /etc/gdb/gdbinit
>   * user-specific init file: /homes/guitton/.gdbinit
>   * local init file: /homes/guitton/work/.gdbinit
> 
> For more information, type "help" from within GDB, or consult the
> GDB manual (available as on-line info or a printed manual).
> [...]
> 
> 
> Would that make sense?

Yes, with changes suggested by Daniel.