Building LDC v1.42.0 on ARM64 OpenBSD

[RubyTheRoobster]

I wanted to use dub, but because there is no dub package for 
arm64 OpenBSD
(only for x64), I needed to build dub from source. After 
installing gdc and getting gdmd
to run on the system, I tried to build dub, only for gdc to keep 
mysteriously
crashing at one specific point. So I decided to try building dub 
using LDC.
But again, there is no ldc package for arm64 OpenBSD, so I had to 
build it from
source. I downloaded the git, checked out v1.42.0, and followed 
the intructions
in the wiki page linked to by the README. The result was cmake 
giving me an error message:
```
CMake Error at runtime/CMakeLists.txt:480 (add_custom_command)
     add_custom_command Wrong syntax. A TARGET or OUTPUT must be 
specified.
```

Is there any way to fix this, or are dlang build systems simply 
not arm64-friendly?