stdbuf: build on Windows (depending on cygwin dll)

[oech3]

Build Windows native stdbuf.exe depending on libstdbuf.dll built for Cygwin. Useful for MSYS2. Closes #12328

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.

[Ecordonnier]

So if I understand correctly, stdbuf.exe will work only with binaries which use the msys-2.0.dll or cygwin1.dll runtime. Normal windows PE programs won't be impacted at all.
The tools in /mingw64/bin (grep, sed, awk, git) are native PE binaries linking against MSVCRT/UCRT.

It would be good to document this use-case / limitation, because AFAIK stdbuf won't have any effect for the vast majority of MSYS2 usage patterns.

[oech3]

It would be good to document this use-case / limitation, because AFAIK stdbuf won't have any effect for the vast majority of MSYS2 usage patterns.

at manpage or --help?

[Ecordonnier]

It would be good to document this use-case / limitation, because AFAIK stdbuf won't have any effect for the vast majority of MSYS2 usage patterns.

at manpage or --help?

As far as I understand, if you edit the ftl files in src/uu/stdbuf/locales, both the man pages and the --help text willbe updated.
I think the documentation of the limitation could go to the ftl files, yes. If it's not possible, I would at least document it with a comment in the code.

[oech3]

I think people not on Windows does not want to see such document. #[cfg] on clap can avoid that. But if I added it to stdbuf-after-help key, *.ftlduplicates many things. What key should I put it on?

[oech3]

It would be nice if we could do partial dll injection on actual Windows.