fix: build and tests on OpenBSD

[knightjoel]

Description of changes:

This PR repairs and tidies up the build and tests on OpenBSD. Testing performed on OpenBSD 7.2/amd64.

100% tests passed, 0 tests failed out of 424

Total Test time (real) =  54.68 sec

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[graebm]

Thanks for the great contribution!

[codecov-commenter]

Codecov Report

Base: 80.79% // Head: 80.79% // No change to project coverage 👍

Coverage data is based on head (255ed4b) compared to base (951b70f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #976   +/-   ##
=======================================
  Coverage   80.79%   80.79%           
=======================================
  Files          53       53           
  Lines        5665     5665           
=======================================
  Hits         4577     4577           
  Misses       1088     1088           

Impacted Files	Coverage Δ
source/posix/thread.c	80.70% <ø> (ø)

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