Fix build on OpenBSD

[catap]

Here a two trivial commit which unblock current master on OpenBSD.

[nigoroll]

Related #4103

[catap]

@nigoroll indeed, but @dridi patch doesn't include pthread_set_name_np which is the right name on OpenBSD. Plust it needs to resolve conflicts.

[nigoroll]

@catap I am not against having your suggestions on top of it. This has more to do with agreement within the team and reverting the appropriate change.

[dridi]

@nigoroll indeed, but @dridi patch doesn't include pthread_set_name_np which is the right name on OpenBSD. Plust it needs to resolve conflicts.

My focus was not on restoring the previous code that supported multiple non-portable pthread functions. The purpose of my patch was to bring back ifdef guards against a non-portable function just in case it wouldn't exist on some systems. I turned out to be on the right track.

[catap]

@dridi thus, here a PR which is needed to be able to build varnish on OpenBSD again.

[nigoroll]

@bsdphk can we leave the final verdict with you?

[catap]

I had just rebased this PR into the last master.

Also, this fixes required for build Varnish on OpenBSD.