Another option is wolfSSL (https://wolfssl.com/wolfSSL/Home.html) which is GPL-compatible, but also has a commercial license option. They have an OpenSSL compatibility layer, but are not a derivative of OpenSSL.
My experience with their software has been very positive, and they have avoided the majority of recent insecurities. Plus they have great support for anyone working on open source projects.
Your recommendation for TLS elsewhere in the thread was:
> You should use BoringSLL, LibreSSL, Go crypto/tls, or OpenSSL, in roughly that order.
Three of those are based on OpenSSL, and Go crypto/tls presumably only works with Go.