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
diff --git a/lib/Makefile.am b/lib/Makefile.am
index 58ef555..1045237 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -1,6 +1,5 @@
 SUBDIRS = \
-  cpp \
-  py
+  cpp
 
 if ENABLE_MONO
 SUBDIRS += csharp
@@ -10,6 +9,10 @@
 SUBDIRS += java
 endif
 
+if ENABLE_PYTHON
+SUBDIRS += py
+endif
+
 if ENABLE_ERLANG
 SUBDIRS += erl
 endif