main/tcptls: Add checks for OpenSSL Elliptic Curve support

The patch for ASTERISK-23905 that added PFS support in Asterisk depends on the
elliptic curve library support being present in OpenSSL. As it turns out, some
versions of OpenSSL don't have this library - notably the version running on
our build agents.

This patch fixes the build by providing a configure check for the specific
library calls that the PFS patch relies on.

Review: https://reviewboard.asterisk.org/r/3709/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417900 65c4cc65-6c06-0410-ace0-fbb531ad65f3