rpm: do not evaluate python lib path over 100 times

Do not evaluate python_sitearch every time it's used. Override
definition in /usr/lib/rpm/macros.

rpm_spec/qmgr.spec

@@ -1,4 +1,4 @@
-%{!?python_sitearch: %define python_sitearch %(%{__python} -c "from distutils.sysconfig import get_python_lib; print get_python_lib(1)")}
+%global python_sitearch %(%{__python} -c "from distutils.sysconfig import get_python_lib; print get_python_lib(1)")
 
 %{!?version: %define version %(cat version)}