[Bug other/91879] --with-build-time-tools doesn't work as expected

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91879

--- Comment #28 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to joseph@codesourcery.com from comment #22)
> The build system design is that where A and B are both built at the same 
> time, and the build of B uses A, it should use the *newly built* copy of A 
> as long as that is for _$host = $build_.

Indeed! I missed this "$host = $build" part initially.
When I build djgpp that _runs_ under DOS, then it is not
used during the compilation, as that would simply be impossible.
So such config is already supported.
Is there any way to convince the build system that the
resulting compiler is alien and cannot be used? I think
$host = $build check is just insufficient; there may be
more cases when the resulting compiler can't be used on
a build system, like different prefix.

If there is no such option currently, what do you think
about replacing the
if test "${build}" != "${host}"
with
if test "${build}" != "${host}" -o "$non_native_build"
or something like this?