Support OpenBSD
It would be nice if OpenBSD was supported.

As far as I can see adding `or defined(OpenBSD)` to the branch containing `FreeBSD` should do the trick. I've used this to successfully install and use [monocypher](https://github.com/markspanbroek/monocypher.nim).

Unfortunately nimterop's tests fail with the following:

```
Running: nim c --hints:off -f -d:checkAbi -r tests/tnimterop_c.nim

# Importing /home/user/repos/nimterop/tests/include/test.h
# Saved to /home/user/.cache/nim/nimterop/toastCache/nimterop_3018790.nim
/home/user/repos/nimterop/tests/tnimterop_c.nim(52, 9) Error: undeclared identifier: 'OSDEF'
candidates (edit distance, scope distance); see '--spellSuggest':
 (3, 4): 'DEF' [type declared in /home/user/.cache/nim/nimterop/toastCache/nimterop_3018790.nim(189, 3)]
 (3, 6): 'GC_ref' [proc declared in /home/user/.nim/nim-1.6.4/lib/system/gc_interface.nim(44, 8)]
 (3, 6): 'GC_ref' [proc declared in /home/user/.nim/nim-1.6.4/lib/system/gc_interface.nim(45, 8)]
 (3, 6): 'GC_ref' [proc declared in /home/user/.nim/nim-1.6.4/lib/system/gc_interface.nim(46, 8)]
 (3, 6): 'dec' [proc declared in /home/user/.nim/nim-1.6.4/lib/system/arithmetics.nim(31, 6)]
 (3, 6): 'del' [proc declared in /home/user/.nim/nim-1.6.4/lib/core/macros.nim(221, 6)]
 (3, 6): 'del' [proc declared in /home/user/.nim/nim-1.6.4/lib/core/macros.nim(221, 6)]
 (3, 6): 'del' [proc declared in /home/user/.nim/nim-1.6.4/lib/system.nim(1320, 6)]
 (3, 6): 'ref' [type declared in /home/user/.nim/nim-1.6.4/lib/system.nim(43, 3)]
 (3, 6): 'seq' [type declared in /home/user/.nim/nim-1.6.4/lib/system.nim(299, 3)]
 (3, 6): 'set' [type declared in /home/user/.nim/nim-1.6.4/lib/system.nim(300, 3)]

Ran: nim c --hints:off -f -d:checkAbi -r tests/tnimterop_c.nim
Time taken: 19 seconds, 60 milliseconds, 362 microseconds, and 998 nanoseconds

stack trace: (most recent call last)
/tmp/nimblecache-2194727937/nimscriptapi_1651064302.nim(187, 16)
/home/user/repos/nimterop/nimterop.nimble(99, 12) testTask
/home/user/repos/nimterop/nimterop.nimble(62, 12) basicTask
/home/user/repos/nimterop/nimterop.nimble(21, 11) execTest
/home/user/repos/nimterop/nimterop.nimble(18, 8) execCmd
/home/user/.nim/nim-1.6.4/lib/system/nimscript.nim(273, 7) exec
/home/user/.nim/nim-1.6.4/lib/system/nimscript.nim(273, 7) Error: unhandled exception: FAILED: tests/timeit nim c --hints:off -f -d:checkAbi  -r tests/tnimterop_c.nim [OSError]
     Error: Exception raised during nimble script execution
```
