Only attempt to change the allowed TLS version range if it's supported.
The necessary functions were added in NSS 3.14.
Hopefully fixes the Jenkins build on debian-ppc64, which is apparently
using a pretty old version of NSS?
https://www.guifications.org/jenkins/view/Pidgin/job/pidgin-debian-ppc64-2.x.y/
Thanks to Tomasz for noticing that it was broken.