Adds xpyb support when building with waf
---
--- pycairo-1.10.0/wscript	2011-04-18 09:42:29.000000000 +0200
+++ pycairo/wscript	2012-11-12 08:37:20.000000000 +0100
@@ -9,6 +9,7 @@
 APPNAME='pycairo'
 VERSION='1.10.0'
 cairo_version_required = '1.10.0'
+xpyb_version_required  = '1.3'    # optional
 
 
 def options(ctx):
@@ -27,9 +28,17 @@
   ctx.check_tool('python')
   ctx.check_python_version((3,1,0))
   ctx.check_python_headers()
+
   ctx.check_cfg(package='cairo', atleast_version=cairo_version_required,
                 args='--cflags --libs')
 
+# xpyb for Python 3 is not available yet.
+# the Python 3 version should probably have a different name than 'xpyb'
+#  ctx.check_cfg(package='xpyb',
+#                atleast_version=xpyb_version_required,
+#                args='--cflags --libs',
+#                mandatory=False)
+
   # add gcc options
   if env['CC_NAME'] == 'gcc':
     env.append_unique('CCFLAGS', ['-std=c99', '-Wall'])