pythons built with pyenv's python-build leak CFLAGS/LDFLAGS into user packages via sysconfig

These flags will end up in builds of user Python packages with extensions
via sysconfig/distutils.

Specifically, as per https://trac.sagemath.org/ticket/32531#comment:3, the built pythons have -I and -L options in the sysconfig variables CFLAGS and LDFLAGS.

This should be fixed by putting the necessary flags for the Python build
into CFLAGS_NODIST and LDFLAGS_NODIST instead -- see similar fix for Homebrew's packaging of
python3 in Homebrew/homebrew-core#68528
and Cygwin's packaging of python3 (https://cygwin.com/pipermail/cygwin/2021-December/250302.html).

Make sure your problem is not listed in the common build problems.
Make sure no duplicated issue has already been reported in the pyenv issues. You should look for closed issues, too.
Make sure you are not asking us to help solving your specific issue.
- GitHub issues is opened mainly for development purposes. If you want to ask someone to help solving your problem, go to some community site like Gitter, StackOverflow, etc.
Make sure your problem is not derived from packaging (e.g. Homebrew).
- Please refer to the package documentation for the installation issues, etc.
Make sure your problem is not derived from plugins.
- This repository is maintaining pyenv and the default python-build plugin only. Please refrain from reporting issues of other plugins here.

Platform information (e.g. Ubuntu Linux 16.04): all
OS architecture (e.g. amd64): all
pyenv version: HEAD
Python version: all
C Compiler information (e.g. gcc 7.3): irrelevant
Please attach the debug trace of the failing command as a gist:
- Run env PYENV_DEBUG=1 <faulty command> 2>&1 | tee trace.log and attach trace.log. E.g. if you have a problem with installing Python, run env PYENV_DEBUG=1 pyenv install -v <version> 2>&1 | tee trace.log (note the -v option to pyenv install).

Provide feedback