SecureTransport is a system library on Apple operating systems that implements TLS. It doesn’t exist on Linux. The configure help output is showing it on Linu

SecureTransport is a system library on Apple operating systems that implements TLS. It doesn’t exist on Linux. The configure help output is showing it on Linux because it doesn’t filter the list of options based on the OS you’re running on. (Nor should it, since you might hypothetically want to cross-compile a binary for a different operating system than the one you’re running on.)