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This fix relies on having the 'fchdir' function, and on having
"." be searchable (or at least readable, on platforms lacking O_SEARCH),
but that's good enough to handle the vast majority of cases and the
remaining folks can just live with the annoyance of file systems
that occasionally can't be unmounted.
* configure.ac (fchdir): New function to check for.
* lib/save-cwd.c: Copy from gnulib, except omit the part that
allocates memory, since that can cause problems in Emacs.
* lib/save-cwd.h: Copy from gnulib.

Fixes: debbugs:18232