The pyconfig.h and makefile are no longer used directly. Rather
configuration is baked into `.py` files.

As of Python 3.x, the config header and makefile are not longer used.  Trying to
include them causes build errors in pyenv/venv/equivalents.  Applying this fixes
5018, fixes 4775, and closes 1545 (a cleanup issue).

On Windows:
 The pyconfig.h data is hardcoded directly into sysconfig.py.
 And the makefile does not exist.
On Unix:
 A Python extension module contains all the details. This module is
 a hidden import since 3.6 but is already marked as such in the hook.

The functions to find and parse the files are still left over in sysconfig.py
but they are almost fully replaced by functions to get configuration parameters
directly. Should a user attempt to invoke the parser in a PyInstaller build, it
will now fail. They should instead use get_config_var() which will always work
(and is the recommended usage anyway).

Locating `pyconfig.h` and the `makefile` gets into a mess when using
certain environment managers (pyenv-virtualenv) because PyInstaller,
whilst trying to find an appropriate `dest` path to put the files in,
gets confused by `sys.prefix` (or its varients) not being a parent dir of
the config and makefiles.

As of Python 3.6, direct parsing of `pyconfig.h` and `makefile` are
replaced by a Python module generated on building Python so that
these files are no longer required.