Some other minor updates in _warnings.c:
- make a function static
- rename a shadowing local variable

better performance.

number of splits.

Windows.

when it is run.

length of the list storing the warning options.

gcc's profile guided optimization.

'warnings' code in places where it was previously not possible (e.g., the
parser). It could also potentially lead to a speed-up in interpreter start-up
if the C version of the code (_warnings) is imported over the use of the
Python version in key places.

Closes issue #1631171.

the all_errors tuple.

-J and -X are now reserved for Jython and non-standard arguments (e.g. IronPython). I've added some extra comments to make sure the reservation don't get missed in the future.

PyIndex_Check() was introduced in Python 2.5.