ECDH performance warning with LibreSSL

[notice] Tor 0.3.0.10 (git-c33db290a9d8d0f9) running on Linux with Libevent 2.1.8-stable, OpenSSL LibreSSL 2.5.5 and Zlib 1.2.11.

[notice] We were built to run on a 64-bit CPU, with OpenSSL 1.0.1 or later, but with a version of OpenSSL that apparently lacks accelerated support for the NIST P-224 and P-256 groups. Building openssl with such support (using the enable-ec_nistp_64_gcc_128 option when configuring it) would make ECDH much faster.