Hi Reuben,

> Unfortunately, this doesn't help, because pathconf is not present on mingw.

Then use some fallback, e.g. 256 or PATH_MAX.

> So the best I can do is #define _POSIX_ to get the value of NAME_MAX that
> it has.

Speaking for Gnulib, we cannot support _POSIX_ on mingw. This macro modifies
the behaviour of the header files in 23 places. We already have enough work
with the various header files origins and modes (from Microsoft, with clang,
from old mingw, from mingw64, UNICODE or not,__USE_MINGW_ANSI_STDIO,
_USE_32BIT_TIME_T). We do not need yet another variant, _POSIX_.