python-mapnik: fix pycairo support

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).

pkgs/development/python-modules/python-mapnik/default.nix

@@ -35,10 +35,16 @@ in buildPythonPackage rec {
     export BOOST_PYTHON_LIB="boost_python${pythonVersion}"
     export BOOST_THREAD_LIB="boost_thread"
     export BOOST_SYSTEM_LIB="boost_system"
+    export PYCAIRO=true
   '';
 
   nativeBuildInputs = [
     mapnik # for mapnik_config
+    pkgs.pkgconfig
+  ];
+
+  patches = [
+    ./find-pycairo-with-pkg-config.patch
   ];
 
   buildInputs = [