[master] 9305a2d pull out vcs_version.h / vmod_abi.h generation to own file again

[Dridi Boukelmoune]

> d-oh, I was not aware of that use case - sorry.
>
> You mentioned you wanted to look after this - please keep in mind that if you
> _do_ have a dist, vcs_version should not be touched, iow, you should not get
> NOGIT when building dist.

Yes, I have a fix covering all 3 cases and will submit it for review.

Cheers