implement CreateThread with boost::thread

I'm not sure why this wasn't done before.

- Removes typedef of pthread_t on Windows, which fixes a native compile issue on mingw.

src/util.cpp

@@ -1299,3 +1299,15 @@ void RenameThread(const char* name)
     (void)name;
 #endif
 }
+
+bool CreateThread(void(*pfn)(void*), void* parg)
+{
+    try
+    {
+        boost::thread(pfn, parg); // thread detaches when out of scope
+    } catch(boost::thread_resource_error &e) {
+        printf("Error creating thread: %s\n", e.what());
+        return false;
+    }
+    return true;
+}