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

[David Holmes]

On Wed, 25 Aug 2021 08:51:11 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> 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.
> 
> Fair enough.

@jerboaa , @iklam  again thanks for looking at this, but can someone actually approve it please :) Thanks.

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