Dmitry V. Levin wrote:
> Is this __DEFINED_va_list macro the official way of detecting musl?

No, but in a world where the musl people don't want to provide an official
way [1][2] and the Alpine Linux people break their previously working way of
detecting musl [3], we (GNU) need to use our own heuristics to fulfil the
practical need of programs (especially test suites) to distinguish musl
systems from glibc systems.

> It looks very fragile to me.

Indeed, config.guess would be less fragile if we combined both heuristics.
Patch attached. It's a trade-off between code size and robustness.

Bruno

[1] https://wiki.musl-libc.org/faq.html
[2] https://lists.gnu.org/archive/html/bug-gnulib/2018-02/msg00079.html
[3] https://lists.gnu.org/archive/html/config-patches/2020-09/msg00002.html