In build_extensions(), don't proceed if srcdir is None. Probably
somebody who tried this on Windows. :-)
# Fix up the autodetected modules, prefixing all the source files
# with Modules/ and adding Python's include directory to the path.
(srcdir,) = sysconfig.get_config_vars('srcdir')
+ if not srcdir:
+ # Maybe running on Windows but not using CYGWIN?
+ raise ValueError("No source directory; cannot proceed.")
# Figure out the location of the source code for extension modules
moddir = os.path.join(os.getcwd(), srcdir, 'Modules')