This allows building without SCTP support (and even building/running tests). The "HAVE_SCTP" define has been functional for a while, but there wasn't any easy way to turn it on/off. NOTRY=True BUG=webrtc:6933 Review-Url: https://codereview.webrtc.org/2593313002 Cr-Commit-Position: refs/heads/master@{#15763}