RFR: 8272811: Document the effects of building with _GNU_SOURCE in os_posix.hpp

[Ioi Lam]

On Mon, 23 Aug 2021 04:27:03 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Please review this trivial code documentation enhancement based on discussions here:
> 
> http://mail.openjdk.java.net/pipermail/jdk-dev/2021-August/005946.html
> 
> Thanks,
> David

Marked as reviewed by iklam (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/5212