[Ada] Don't unconditionally define _LARGEFILE_SOURCE

[Arnaud Charlet]

> 2014-08-01  Pascal Obry  <obry@adacore.com>
> 
>         * cstreams.c: Only enable large file support on know supported
>         platforms. Add missing defines/includes.
> 
> broke Ada bootstrap on Solaris:
> 
> /vol/gcc/src/hg/trunk/local/gcc/ada/cstreams.c:34:0: error:
> "_LARGEFILE_SOURCE" redefined [-Werror]
>  #define _LARGEFILE_SOURCE
>  ^
> <built-in>: note: this is the location of the previous definition
> 
> Fixed as follows, i386-pc-solaris2.10 bootstrap is past the failure now.
> 
> Ok for mainline?

Yes, thanks.