[Distutils] optional binary compilation under future installation tools

[Chris McDonough]

Some packages we maintain currently provide largely identical
side-by-side implementations of features: one implementation is written
in C, the other implementation is written in Python.  The C module is
just an optimized version of the Python code.

There is logic at module scope within modules in the same package which
attempts to import the C version.  If the C version cannot be imported
(an ImportError is raised), the logic falls back to importing the Python
version:

try:
    from . import cmodule as module
except ImportError
    from . import pymodule as module

This means that the package can be used if the C implementation is
compiled or not.  It will run more slowly, but that's OK in lots of
cases.

In our setup.py for these kinds of distributions, we have code which
under Py2 uses a setuptools "feature" and under Py3 registers a
different kind of build_ext which, at install time, will:

- Attempt to compile the C if suitable build tools seem to exist on the
  target system.

- If suitable build tools don't seem to exist on the target system, will
  print a message and continue.

How can we support such a feature in the brave new declarative packaging
world?

- C