gcc.dg/compat/struct-layout-1.exp does not supported installed-compiler testing

[DJ Delorie]

> (There's still a POSIX-ism in the generator, in that it tries to
> write to "/dev/null".  On Windows systems, I bet this will often
> work, but create a real file with that name.  It would be better,
> and avoid portability problems, to guard the calls to fwrite, etc.,
> with "if (file)" rather than spew to "/dev/null", but that's for
> another day.)

Both Cygwin and DJGPP know about /dev/null just fine.  I don't know
about MinGW though.

But it's a lot faster if you don't do the write at all ;-)