On systems where 'mode_t' is smaller than 'int', doing 'va_arg (ap, mode_t)' is
wrong because of C's "default argument promotions". GCC 4 creates crashing code
in this case.
The "va_arg" page of Gnulib's manual describes the problem and a simple solution:
https://www.gnu.org/software/gnulib/manual/html_node/va_005farg.html
However, since that solution reportedly (see thread at next link) still causes
GCC to print warnings (for no good reason; perhaps this was fixed in newer
GCCs), we pick a solution that defines a PROMOTED_MODE_T at the configuration
stage:
https://lists.gnu.org/archive/html/bug-gnulib/2009-05/msg00231.html
(We take our 'mode_t.m4' from the most recent Gnulib source.)
(If any of the URLs above no longer works, simply search the web for the
mentioned words.)
