Linking mimalloc-test-stress-dynamic fails on OpenBSD

Nobody has claimed this yet.

Commit 07743454e5a04356144918e32b1e2ce8e80c1726 breaks `mimalloc-test-stress-dynamic` linking:

```
[100%] Linking C executable mimalloc-test-stress-dynamic
ld: warning: test-stress.c(CMakeFiles/mimalloc-test-stress-dynamic.dir/test/test-stress.c.o:(main)): warning: rand() may return deterministic values, is that what you want?
ld: error: undefined symbol: pthread_create
>>> referenced by test-stress.c
>>> CMakeFiles/mimalloc-test-stress-dynamic.dir/test/test-stress.c.o:(main)

ld: error: undefined symbol: pthread_join
>>> referenced by test-stress.c
>>> CMakeFiles/mimalloc-test-stress-dynamic.dir/test/test-stress.c.o:(main)
cc: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error 1 in . (CMakeFiles/mimalloc-test-stress-dynamic.dir/build.make:101 'mimalloc-test-stress-dynamic': /usr/local/bin/cmake -E cmake_l...)
*** Error 2 in . (CMakeFiles/Makefile2:332 'CMakeFiles/mimalloc-test-stress-dynamic.dir/all': make -s -f CMakeFiles/mimalloc-test-stress-dyn...)
*** Error 2 in /home/test/mimalloc/out/release (Makefile:146 'all': make -s -f CMakeFiles/Makefile2 all)
```

Start with test/test-stress.c and the CMake build configuration introduced or changed by commit 07743454e5a04356144918e32b1e2ce8e80c1726. Reproduce the dynamic stress-test build on OpenBSD and inspect the link command for the missing pthread symbols. Done means mimalloc-test-stress-dynamic links successfully on OpenBSD without breaking other builds.

Written by the indexing model from the issue text.