`uv_thread_self()` works on Windows only for threads created using
`uv_thread_start()` because libuv does not use `GetCurrentThreadId()`
for threads that were created otherwise. `uv_thread_equal()` works
correctly.

Thus, on Windows we use `GetCurrentThreadId()` to compare the main
thread with the current thread.

Signed-off-by: Gabriel Schulhof <gabriel.schulhof@intel.com>

Co-Authored-By: Anna Henningsen <github@addaleax.net>