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/lib/py/Makefile.am b/lib/py/Makefile.am
index 24c5892..7db5c3b 100644
--- a/lib/py/Makefile.am
+++ b/lib/py/Makefile.am
@@ -5,4 +5,4 @@
 # Old version (can't put inline because it's not portable).
 #$(PYTHON) setup.py install --prefix=$(prefix) --root=$(DESTDIR) $(PYTHON_SETUPUTIL_ARGS)
 install-exec-hook:
-	$(PYTHON) setup.py install --root=$(DESTDIR) $(PYTHON_SETUPUTIL_ARGS)
+	$(PYTHON) setup.py install --root=$(DESTDIR) --prefix=$(PY_PREFIX) $(PYTHON_SETUPUTIL_ARGS)