Allow Thrift to be built without the Python library.

- Modify configure.ac to check for Python conditionally, like we do for
  Java, Mono, and Erlang.
- Modify Makefile.am to add "py" to SUBDIRS only if Python was detected.


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@666488 13f79535-47bb-0310-9956-ffa450edef68
3 files changed