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/configure.ac b/configure.ac
index 9a4aeb5..2605478 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6,6 +6,12 @@
 
 AM_INIT_AUTOMAKE
 
+AC_ARG_VAR([PY_PREFIX], [Prefix for installing Python modules.
+                         (Normal --prefix is ignored for Python because
+                         Python has different conventions.)
+                         Default = "/usr"])
+AS_IF([test "x$PY_PREFIX" == x], [PY_PREFIX="/usr"])
+
 AC_CONFIG_FILES([Makefile])
 
 AC_CONFIG_SUBDIRS([compiler/cpp lib/cpp lib/py if])