LibreSSL is lacking features such as ALPN and they've removed many constants, changed function definitions in subtle ways and modified header include dependencies. The result of this is that it definitely isn't a drop in replacement for OpenSSL. Then again OpenSSL usually isn't a drop in replacement for OpenSSL between versions either so they aren't doing a terrible job.

Most of these could be easily worked around with a few #ifdefs but they've also managed to make that a bit problematic by reusing the OPENSSL_VERSION_NUMBER macro without providing some sort of complementary IS_LIBRESSL flag. Fortunately OpenSSL hasn't hit version 2 yet so the version numbers don't overlap at all.