- The things_to_do static variable should be declared as "volatile": it
is read by the main loop without any lock.
(by the way, could you rename it to something like "pendingcalls_to_do"?)

- in the old Py_MakePendingCalls function, the "#ifdef WITH_THREAD" part
is not useful now. 

- the "XXX" comments should probably be rewritten.

A note to people concerned by performance: the additional lock is used
only when there is an actual pending call to process. The main loop only
regularly checks the "things_to_do static" volatile static variable, and
the patch does not change this.