When it comes to using cinterop with pure-header libraries like stb_image.h, do I need to make a file like placeholder.c that just includes the header? Or can I just do #include in the interop.def file? I've run into some problems on both, so I'm not sure which obstacle I should try to surmount.

You don’t need to create anything to interop with such libs, for example

cinterop -h stb_image.h -compilerOpts "-I. -DSTB_IMAGE_IMPLEMENTATION=1" -pkg stb_image

produces

nativelib.klib

, you can examine its contents with

klib contents nativelib.klib

Much obliged.