Avoid trying to link against the pthreads library when building on Win32

src/CMakeLists.txt

@@ -35,10 +35,12 @@ target_link_libraries(updatershared
   tinyxml
   minizip
   tinythread
-  # TODO - Mac and Windows version
-  pthread
 )
 
+if (UNIX)
+  target_link_libraries(updatershared pthread)
+endif()
+
 add_executable(updater
   main.cpp
 )