Our engineering team has done a bit more digging into libwebsockets and has informed me that the unmodified version of libwebsockets 1.7.4 is in fact also used in UE. This means that in both UE 4.15.2 and 4.16 the modified libwebsockets 1.7.3 and the unmodified libwebsockets 1.7.4 are statically linked.