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

[David Holmes]

On Tue, 24 Aug 2021 16:58:00 GMT, Severin Gehwolf <sgehwolf 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
>
> Shouldn't something along those lines also be mentioned where it's actually defined? Like:
> https://github.com/openjdk/jdk/blob/d34f17c6973748693de1bdd040bc3e8a0f15f197/make/autoconf/flags-cflags.m4#L432

@jerboaa  thanks for taking a look. I initially planned to document it in the makefiles somewhere but then found that we don't document why we use any flags there :(  Plus there are a number of places where -D_GNU_SOURCE is used and I only want to document the reasoning for the VM which would then beg the question is to why the other uses are not documented somehow. So I just put it in os_posix.hpp where it was more likely to be seen when needed.

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