Using OpenSSL from a subproject

[guybedford]

When trying to include OpenSSL from a subproject via, I'm getting an error for these includes, for example:

#include "openssl/evp.h"

gives that the include is not found.

Can we make it a feature of the subproject build that the OpenSSL headers are reachable? My CMake knowldge is fairly limited here - what would be involved to implement that?