Add support for --with-system-gdbinit

[Jerome Guitton]

Daniel Jacobowitz (drow@false.org):

> On Mon, Jan 26, 2009 at 10:12:34AM +0100, Jerome Guitton wrote:
> > 2009-01-26  Daniel Jacobowitz  <dan@codesourcery.com>
> > 	    Jerome Guitton  <guitton@adacore.com>
> > 
> > 	* configure, config.in: Regenerated.
> > 	* configure.ac: Add --with-system-gdbinit.
> > 	* main.c (get_init_files): New.
> > 	(captured_main): Use get_init_files.  Load system gdbinit before
> > 	$HOME/.gdbinit.
> > 	(print_gdb_help): Print location of init files.
> 
> This version looks OK to me.  Thanks!

Now committed! Thank you.