OpenSSL is pretty sane when it comes to its threading
implementation. It doesn't depend on any particular
threading library, but rather depends on callbacks you
register with it. If you don't register the callbacks,
threading isn't enabled. It doesn't link to any thread
Now, because it is sane, it makes sense to just enable
the thread support all the time, otherwise, the interfaces
for registering those callbacks don't exist, and applications
which rely on threading and openssl will crash (in a very
hard-to-debug manner) when a concurrency issue is hit.
Here is a patch to enable thread support in OpenSSL.