Fully enable pycairo support by exporting PYCAIRO=true and using
pkg-config to locate the pycairo library (with included patch).
The patch restores the use of pkg-config to find pycairo. This code was
present upstream, but commented out; it has been re-enabled and
modified to support both pycairo and py3cairo (the python3 version
of pycairo).
