Thrift: Python installation.

Summary:
Added the configure script variable PY_PREFIX which serves the same purpose
for Python as --prefix does for C/C++.  We chose to do this because Python
has different conventions for where to install libraries.

Reviewed By: mcslee

Test Plan:
Full clean builds and installs of Thrift from the Thrift root,
with and without PY_PREFIX.  Watched output of make install.

Ran configure from lib/py without PY_PREFIX, with PY_PREFIX and with
PY_PREFIX in the environment.  Checked config.status for each.

Revert Plan: okay


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665248 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/README b/README
index 0953f37..5dc232e 100644
--- a/README
+++ b/README
@@ -108,7 +108,9 @@
 
 Please be aware that the Python library will ignore the --prefix option
 and just install wherever Python's distutils puts it (usually along
-the lines of /usr/lib/pythonX.Y/site-packages/).  DESTDIR is respected.
+the lines of /usr/lib/pythonX.Y/site-packages/).  If you need to control
+where the Python modules are installed, set the PY_PREFIX variable.
+(DESTDIR is respected for Python and C++.)
 
 Make thrift: