http: fix compilation with LibreSSL
http: fix compilation with LibreSSL

LibreSSL defines OPENSSL_VERSION_NUMBER to 0x20000000L, but its API is
compatible with OpenSSL 1.0.1.
Therefore redefine OPENSSL_VERSION_NUMBER to 0x1000115fL (1.0.1u) if
LibreSSL is used.

Fixes: #2327

xiph/opusfile,xiph/opusfile,xiph/opusfile