Fix possible compile error after switch to pthread_sigmask.

CMakeLists.txt

@@ -169,7 +169,7 @@ else ()
     add_definitions(-DBADVPN_THREADWORK_USE_PTHREAD)
     add_definitions(-DBADVPN_THREAD_SAFE=1)
 
-    link_libraries(rt)
+    link_libraries(rt pthread)
 
     if (EMSCRIPTEN)
         add_definitions(-DBADVPN_EMSCRIPTEN)

threadwork/CMakeLists.txt

@@ -1,6 +1 @@
-set(BADVPN_THREADWORK_EXTRA_LIBS)
-if (BADVPN_THREADWORK_USE_PTHREAD)
-    list(APPEND BADVPN_THREADWORK_EXTRA_LIBS pthread)
-endif ()
-
-badvpn_add_library(threadwork "system" "${BADVPN_THREADWORK_EXTRA_LIBS}" BThreadWork.c)
+badvpn_add_library(threadwork "system" "" BThreadWork.c)