build: fix windows smp build with gyp

Gyp will try a parallel build if it detect the system has >1 processor.
This functionality depends on the Python "multiprocessing" package. The
multiprocessing package on Windows requires that the top-level module is
importable as a module, see:

  http://docs.python.org/2/library/multiprocessing.html#windows

This fixes issue #984.

This is a back-port of commit 2445467 from the master branch.