stdbuf: support cross-platform building

[YumeYuka]

This PR fixes e.g. building Android targets on Windows.

Because build.rs uses #[cfg] to determine, the suffix of the platform to be built cannot be correctly determined during cross-platform building.

After changing from #[cfg] to env::var, feat_os_unix_android, which includes stdbuf, can be compiled normally. There is no actual test for other cross-platform compilation scenarios, but in theory it can be used normally.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!

[Ecordonnier]

Can you please add a PR description and clarify the functional fix?

[YumeYuka]

Can you please add a PR description and clarify the functional fix?

Sorry a description has now been added.