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 = [

pkgs/development/python-modules/python-mapnik/find-pycairo-with-pkg-config.patch

@@ -0,0 +1,18 @@
+diff --git a/setup.py b/setup.py
+index 82a31d733..1c876a553 100755
+--- a/setup.py
++++ b/setup.py
+@@ -228,10 +228,9 @@ extra_comp_args = list(filter(lambda arg: arg != "-fvisibility=hidden", extra_co
+ if os.environ.get("PYCAIRO", "false") == "true":
+     try:
+         extra_comp_args.append('-DHAVE_PYCAIRO')
+-        print("-I%s/include/pycairo".format(sys.exec_prefix))
+-        extra_comp_args.append("-I{0}/include/pycairo".format(sys.exec_prefix))
+-        #extra_comp_args.extend(check_output(["pkg-config", '--cflags', 'pycairo']).strip().split(' '))
+-        #linkflags.extend(check_output(["pkg-config", '--libs', 'pycairo']).strip().split(' '))
++        pycairo_name = 'py3cairo' if PYTHON3 else 'pycairo'
++        extra_comp_args.extend(check_output(["pkg-config", '--cflags', pycairo_name]).strip().split(' '))
++        linkflags.extend(check_output(["pkg-config", '--libs', pycairo_name]).strip().split(' '))
+     except:
+         raise Exception("Failed to find compiler options for pycairo")
+