[jdk17u-dev] RFR: 8272811: Document the effects of building with _GNU_SOURCE in os_posix.hpp

[Matthias Baesken]

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

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Commit messages:
 - Backport c4c76e2f34f7e709f8b3c960d8c796ff4e2dc4a5

Changes: https://git.openjdk.org/jdk17u-dev/pull/2164/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2164&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8272811
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2164.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2164/head:pull/2164

PR: https://git.openjdk.org/jdk17u-dev/pull/2164