Tried to build curl with PolarSSL on Ubuntu 14.04 Trusty Tahr. It compiled, but linking failed because of an undefined reference to ssl_session_init which was introduced in 9f498de and is only available in PolarSSL versions 1.3.8 and 1.3.9. Ubuntu Trusty comes with PolarSSL 1.3.4.
I expected the following
Either the minimum version documented in INTERNALS.md bumped to 1.3.8 and the minimum POLARSSL_VERSION_NUMBER in polarssl.c bumped to 0x01030800 or a successful build.
I believe commit 04b4ee5 (June 2016) is what broke support for older versions, and that change doesn't look like a quick and easy fix to get working for older PolarSSL versions. The fact that it took 10 months for someone to report it could also work as a sign...