This seems to only be used with the OpenSSL backend (which we plan to
drop).
libgit2 already use builtin ZLIB, I'm not even sure we if it would be
used in libssh2 (we only use libssh2 to provide git ssh access, and git
already compress objects.
libgit2 CMake configuration uses a custom flag `STATIC_CRT` and not the
new CMake policy CMP0091.
When forcing the CMake policy (as we do in our cmake tool), we need to
also set the custom `STATIC_CRT` to avoid compiler warning about the /MD
and /MT flag being overridden.
