The cause of CDRIVER-3541 appears that libmongoc detects an installation of LibreSSL, but interprets it as OpenSSL since it configures with ENABLE_SSL=AUTO. LibreSSL intends to be API compatible with OpenSSL, so I think this is intended behavior (i.e. if a user does not explicitly configure with ENABLE_SSL=LIBRESSL, compile against our OpenSSL code). Our docs also indicate such:

The MongoDB C Driver supports LibreSSL through the use of OpenSSL compatibility checks when configured to compile against openssl. It also supports the new libtls library when configured to build against libressl.

But we do not test compiling against LibreSSL and configuring with ENABLE_SSL=AUTO, which is why we did not detect the bug CDRIVER-3541.

Let's add an evergreen task to test ENABLE_SSL=AUTO and automatically detecting LibreSSL.