BF: use setup.py build_src for "make cython" (Closes: scikit-learn#7873…

…) (scikit-learn#7874)

Makefile

@@ -49,7 +49,7 @@ trailing-spaces:
 	find sklearn -name "*.py" -exec perl -pi -e 's/[ \t]*$$//' {} \;
 
 cython:
-	python build_tools/cythonize.py sklearn
+	python setup.py build_src
 
 ctags:
 	# make tags for symbol based navigation in emacs and vim