cmake: don't use builtin sha1 if not using ws

The builtin SHA1 (ZMQ_USE_BUILTIN_SHA1) is only used in the websocket
engine (ws_engine.cpp), so if websockets are disabled, i.e
-DENABLE_DRAFTS=OFF, don't add sha1.c to the sources list.

CMakeLists.txt

@@ -234,7 +234,7 @@ if(NOT ZMQ_USE_GNUTLS)
       endif()
     endif()
   endif()
-  if(NOT ZMQ_USE_NSS)
+  if(ENABLE_WS AND NOT ZMQ_USE_NSS)
     list(APPEND sources ${CMAKE_CURRENT_SOURCE_DIR}/external/sha1/sha1.c
          ${CMAKE_CURRENT_SOURCE_DIR}/external/sha1/sha1.h)
     message(STATUS "Using builtin sha1")