bpo-45573: Introduce extension module flags in Makefile (GH-29594)

``configure`` now uses a standardized format to forward state, compiler
flags, and linker flags to ``Makefile``, ``setup.py``, and
``Modules/Setup``. ``makesetup`` use the new variables by default if a
module line does not contain any compiler or linker flags. ``setup.py``
has a new function ``addext()``.

For a module ``egg``, configure adds:

* ``MODULE_EGG`` with value yes, missing, disabled, or n/a
* ``MODULE_EGG_CFLAGS``
* ``MODULE_EGG_LDFLAGS``

``Makefile.pre.in`` may also provide ``MODULE_EGG_DEPS`` that lists
dependencies such as header files and static libs.

Signed-off-by: Christian Heimes <christian@python.org>