Run cythonize() in parallel in "new_build_ext" if threaded distutils build was requested.

Cython/Distutils/build_ext.py

@@ -14,9 +14,11 @@ else:
 class new_build_ext(_build_ext, object):
     def finalize_options(self):
         if self.distribution.ext_modules:
+            nthreads = getattr(self, 'parallel', None)  # -j option in Py3.5+
+            nthreads = int(nthreads) if nthreads else None
             from Cython.Build.Dependencies import cythonize
             self.distribution.ext_modules[:] = cythonize(
-                self.distribution.ext_modules)
+                self.distribution.ext_modules, nthreads=nthreads)
         super(new_build_ext, self).finalize_options()
 
 # This will become new_build_ext in the future.