Few filesystem-related tests rely on PATH_MAX for buffers, and PATH_MAX
is optional in POSIX (and not available on the Hurd). To make them build
and pass, provide a fallback definition of PATH_MAX in case not
available.
Ideally speaking, the tests ought to not unconditionally rely on
PATH_MAX, do alternative strategies (e.g. dynamically allocate buffers,
expand them as needed, etc); OTOH this is test code, so it would be more
work that what it would be worth, so IMHO the define fallback is good
enough.
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