Simple cythonize build_ext.

Cython/Build/Distutils.py

+import sys
+from .Dependencies import cythonize
+
+if 'setuptools' is sys.modules:
+    from setuptools.command import build_ext as _build_ext
+else:
+    from distutils.command import build_ext as _build_ext
+
+class build_ext(_build_ext.build_ext):
+    def finalize_options(self):
+        self.distribution.ext_modules[:] = cythonize(
+            self.distribution.ext_modules)
+        super(build_ext, self).finalize_options()

Cython/Build/__init__.py

 from .Dependencies import cythonize
+from .Distutils import build_ext