metalink: allow compiling with multiple SSL backends

Previously, the code assumed that at most one of the SSL backends would
be compiled in, emulating OpenSSL's functions if the configured backend
was not OpenSSL itself.

However, now we allow building with multiple SSL backends and choosing
one at runtime. Therefore, metalink needs to be adjusted to handle this
scenario, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>