enable threads by default on openbsd 6.2 and higher

Evan Hunt requested to merge each-enable-threads into v9_12

Threads seem to work fine on all currently supported versions of openbsd, so let's use them by default in versions 6.2 and up.

(Note, this is only for 9.12 and 9.11; non-threaded builds no longer exist as of 9.13.)

I'm unsure whether this rates a CHANGES note.

Merge request reports