iconv fix for illumos distributions

[jMichaelA]

Extends the ICONV_CONST const preprocessor for iconv to illumos based distributions.

[devnexen]

Yes I looked at it last night, was (pleasantly) surprised that __illumos__ exists ! Will relook a bit later today. Cheers.

[jMichaelA]

Thanks devnexen. I made this for OmniOS, and I ran through a basic installation. Let me know if you have any questions that I can help with!

[devnexen]

The only question I would have .. should solaris have the same treatment ? solaris is still supported until 2027 I believe.

[jMichaelA]

defined(__sun) && defined(__SVR4) works for OmniOS and should work to support more Solaris distros as well. So that would be a good option as well.

[devnexen]

lgtm

[Girgias]

@devnexen are you planning on merging this?

[devnexen]

well I asked RM opinion on this otherwise I ll wait post 8.5

[DanielEScherzer]

well I asked RM opinion on this otherwise I ll wait post 8.5

Sorry for the delay, RM approval, technical review not performed

[petk]

Here, there is also this PR opened #16847

I think adjusting the preprocessor check for all platforms is quite difficult to do correctly. For example, on newer NetBSD versions, const isn't needed anymore #20089

So, I think that configuration phase check should resolve this issue.