libressl: build with cmake

LibreSSL 2.9.1 no longer builds with the default autotools configuration.
When I searched for the error, I noticed that Buildroot ran into the
same issue, and they resolved the problem by building with CMake rather
than autotools. [1] I followed the same approach here.

[1]: https://github.com/buildroot/buildroot/commit/e783d60473944f8b39f1def45d8d6b483a062158