Objects/typeslots.inc is broken, and make stop on cygwin-1.7.35-i686.

If built-in python is not found in make processing, Objects/typeslots.inc generating will fail.
Because Objects/typeslots.inc is generated using $(PYTHON) in Makefile, but  $(PYTHON) variable is a hard-coding value that isn't check a built-in python existent.

make steps to fail and error message:
$ ./configure --prefix=/opt/py34
$ touch Include/typeslots.h
$ make
...
python.exe ./Objects/typeslots.py < ./Include/typeslots.h > Objects/typeslots.inc
/bin/sh: python.exe: command not found
Makefile:851: recipe for target 'Objects/typeslots.inc' failed
make: *** [Objects/typeslots.inc] Error 127

Cygwin python executable files:
$ find /usr/bin/ -name 'python*.exe'
/usr/bin/python2.7.exe
/usr/bin/python3.2m.exe

I wrote the patch to replace $(PYTHON).

Changes:
Add TYPESLOTSGEN to configure.ac.  TYPESLOTSGEN values is the built-in python.
If the built-in python is not found, TYPESLOTSGEN echoes message that is skipping like ASDLGEN.
Generated the configure script using autoreconf.
Modify Makefile.pre.in to replace $(PYTHON) to $(TYPESLOTSGEN).
And remove $(PYTHON) that isn't used anywhere.  It seems already needless.

Although I understand that the generated file touch can avoid to fail regenerate, I want to check built-in python to generate safer the Objects/typeslots.inc.

Sorry nobody acted on your patch earlier. This is the same as Issue 26662, which has a more general patch that does the same sort of thing.