[trivial] Sync ax_pthread with upstream draft4 #8198

Draft4 of ax_pthread has been available since February, and is the current version available from gnu.org. It fixes an issue on FreeBSD, so sync our version with upstream.

Whitespaceless diff

The non-shitespaced diff https://github.com/bitcoin/bitcoin/pull/8198/commits/0e209f9bf940ec2f6215885cf30deaa7355086e9?w=0 shows that only serial is bumped up and freebsd removed from some condition.

utACK https://github.com/bitcoin/bitcoin/pull/8198/commits/0e209f9bf940ec2f6215885cf30deaa7355086e9

But: nobody reported this issue to us anyway. But: why not... I think it makes sense only if we have an issue for such "syncs".

Yes I'm not sure why there is such a whitespace difference between the actual draft4 and the version available on gnu.org. I guess we could just include the important changes?

utACK 0e209f9

But: nobody reported this issue to us anyway. But: why not... I think it makes sense only if we have an issue for such "syncs".

Agree, I don't think we should do this too often, especially not if the specific issue isn't reported to us.

Maybe it would makes sense to schedule an update of the .m4s once per (major) release or so? And group them together in one pull. To include it in the per-release actions at the top of the release process?

Apart from that, it's harmless (the actual diff is minimal and obvious), so utACK 0e209f9