setup: use setuptools on Windows (issue5400)

We've had a long, complicated history with setuptools. We want to
make it the universal default. But when we do, it breaks things.

`python setup.py build` is broken on Windows today. Forcing
the use of setuptools via FORCE_SETUPTOOLS=1 unbreaks things.

Since the previous bustage with making setuptools the default
was on !Windows, it seems safe to move ahead with the setuptools
transition on Windows. So this patch does that.

setup.py

@@ -63,7 +63,10 b' import re'
 import shutil
 import tempfile
 from distutils import log
-if 'FORCE_SETUPTOOLS' in os.environ:
+# We have issues with setuptools on some platforms and builders. Until
+# those are resolved, setuptools is opt-in except for platforms where
+# we don't have issues.
+if os.name == 'nt' or 'FORCE_SETUPTOOLS' in os.environ:
     from setuptools import setup
 else:
     from distutils.core import setup