Drop support for OpenSSL without ECC.
Offshoot of #16034 (moved).
tor should error out at build time (and possibly runtime if we can easily detect it) if elliptic curve cryptography is not available (or the ECDHE suites we want to use are not available).
OPENSSL_NO_EC is the define for the former, and ECC support is available in any version of OpenSSL that we want to support (>= 1.0.0). I think the only people that ship OpenSSL without all the curves available are RedHat (but at least they have some curves now, as opposed to none). I'm personally ok with breaking builds on such systems if they don't give us all the curves we want.