[CMake] ExternalProject can't have interface library as a dependency

[Stephen Kelly]

Andrey Pokrovskiy wrote:

> /* I'm using cmake-3.2.20150331-gb190c. */
> 
> I have a following construction:
> 
> ExternalProject_Add(
>         websockets_ep
>         DEPENDS ev openssl
>         ...)
> 
> But it so happened, that "openssl" is a INTERFACE library.

That sounds odd. What provides it as an INTERFACE library?