Asked for OpenSSL TLS backend, but it wasn't found CMake cannot find your SSL/TLS libraries. By default, libgit2 always builds with HTTPS support, and you are encouraged to install the OpenSSL libraries for your system (eg, apt-get install libssl-dev).
For development, if you simply want to disable HTTPS support entirely, pass the -DUSE_HTTPS=OFF argument to cmake when configuring it.