https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81926
--- Comment #26 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Ehere is this "BUILD_CONFIG" documented?
It's a developer option so it's not documented but in the configure script:
./configure --help
[...]
--with-build-config='NAME NAME2...'
use config/NAME.mk build configuration
