Honor OPENSSL_NO_OCB if OpenSSL was built this way

Setting ossl110 in the BoringSSL build (see #1944) causes rust-openssl
to expect OCB support. However, OpenSSL already has a feature guard for
OCB, which BoringSSL sets. rust-openssl just isn't honoring it.

This fixes building against an OpenSSL built with ./config no-ocb