Silence 'make -s clean'.

Since distutils spam stdout with "I'm running!" and stderr with "I can't remove
a file because it doesn't exist!", just work around them altogether and don't
clean ctypes if they hadn't been built.

* Makefile.in
  (CTYPES_BUILD_OUTPUTS): New
  (clean-ctypes-python): Only run setup.py if a built output exists.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1479326 13f79535-47bb-0310-9956-ffa450edef68