Revert Cython.Distutils.build_ext to use old_build_ext

This gives projects more time to move over, and reduces the number
of changes in the 0.25 series.

Cython/Distutils/build_ext.py

@@ -11,10 +11,13 @@ else:
     from distutils.command.build_ext import build_ext as _build_ext
 
 
-class build_ext(_build_ext, object):
+class new_build_ext(_build_ext, object):
     def finalize_options(self):
         if self.distribution.ext_modules:
             from Cython.Build.Dependencies import cythonize
             self.distribution.ext_modules[:] = cythonize(
                 self.distribution.ext_modules)
         super(build_ext, self).finalize_options()
+
+# This will become new_build_ext in the future.
+from Cython.Distutils.old_build_ext as build_ext

Cython/Distutils/old_build_ext.py

@@ -21,10 +21,11 @@ try:
     frames = inspect.getouterframes(inspect.currentframe(), 2)
     from_setuptools = 'setuptools/extension.py' in frames[2][1]
     from_pyximport = 'pyximport/pyxbuild.py' in frames[1][1]
+    from_cy_buildext = 'Cython/Distutils/build_ext.py' in frames[1][1]
 except Exception:
-    from_setuptools = from_pyximport = False
+    from_setuptools = from_pyximport = from_cy_buildext = False
 
-if not from_setuptools and not from_pyximport:
+if not from_setuptools and not from_pyximport and not from_cy_buildext:
     warnings.warn(
         "Cython.Distutils.old_build_ext does not properly handle dependencies "
         "and is deprecated.")