As others have pointed out, that's not the case. Something you do have to be careful of is the compile options for libwebsockets and what version you're using. Recent-ish version of libwebsockets in particular disable by default some features Mosquitto relies on, so some installations may suffer.

Version 2.1 of Mosquitto will no longer need libwebsockets (although it can still be selected at compile time) and so websockets support will be less dependant on how other libraries are compiled.