Detect presence of SSL_get_current_compression

Detect presence of SSL_get_current_compression

Apparently we still build against OpenSSL so old that it doesn't
have this function, so add an autoconf check for it to make the
buildfarm happy. If the function doesn't exist, always return
that compression is disabled, since presumably the actual
compression functionality is always missing.

For now, hardcode the function as present on MSVC, since we should
hopefully be well beyond those old versions on that platform.