nimterop - sigjmp_buf

[shashlick]

It is mostly coming from 
[here](https://github.com/ArtifexSoftware/mupdf/blob/eac73e6732b8c649f4d3dc5b05ca782848325e8b/include/mupdf/fitz/context.h#L511).
 You can use `cOverride()` to define it as `type sigjmp_buf = object` if you 
don't intend using it. If you do, it will be better to `type sigjmp_buf 
{.importc, header: "<setjmp.h>".} = object`.