Reverts the change to generate different :file:`pyconfig.h` files based on
compiler settings, as it was frequently causing extension builds to break.
In particular, the ``Py_GIL_DISABLED`` preprocessor variable must now always
be defined explicitly when compiling for the experimental free-threaded
runtime. The :func:`sysconfig.get_config_var` function can be used to
determine whether the current runtime was compiled with that flag or not.
