4.2 Project: "@file" support

[Richard Kenner]

    I want GCC to work well for people, no matter what operating system they 
    are using.  This is a feature that everyone who produces Windows-hosted 
    versions of GCC ends up implementing; I'd like to keep us all from 
    having to keep reinventing the wheel.

Indeed there's a quite old version of this that somebody I know wrote that
I kept putting off dealing with ...