Before this patch `setmode()` was not detected with Cygwin/MSYS, because
it's a macro, not a function, and detection is looking for a function.
Switching to symbol detection doesn't work because it mis-detects it on
BSD systems which features a function with the same name but different
functionality and arguments.
Fix it by looking for a `_setmode()` function on Cygwin/MSYS, and use it
if available.
`_setmode()` is recommended over `setmode()` by Windows documentation so
use that on Windows too. It seems to be available on all supported
compilers, so omit detection.
https://learn.microsoft.com/cpp/c-runtime-library/reference/posix-setmodehttps://learn.microsoft.com/cpp/c-runtime-library/reference/setmode
Officially Windows requires argument `_O_BINARY` with an underscore.
`O_BINARY` is also supported but bound to conditions. Continue to use it
for simplicity. Cygwin supports `O_BINARY` (no underscore).
Closes#15169
5e70566094