Don't append NULL when a variable doesn't exist
This caused a glib warning when the following was encountered in
xml2po.pc (and PYTHON_PREFIX wasn't defined):
modesdir=${PYTHON_PREFIX}/lib/python2.7/site-packages/xml2poDue to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
This caused a glib warning when the following was encountered in
xml2po.pc (and PYTHON_PREFIX wasn't defined):
modesdir=${PYTHON_PREFIX}/lib/python2.7/site-packages/xml2po