Adds xpyb support when building with waf
---
--- py2cairo-1.10.0/wscript	2011-05-01 13:01:22.000000000 +0200
+++ py2cairo/wscript	2012-11-12 08:39:19.000000000 +0100
@@ -9,6 +9,7 @@
 APPNAME='py2cairo'
 VERSION='1.10.0'
 cairo_version_required = '1.10.0'
+xpyb_version_required  = '1.3'
 
 
 def options(ctx):
@@ -27,9 +28,13 @@
   ctx.check_tool('python')
   ctx.check_python_version((2,6,0))
   ctx.check_python_headers()
+
   ctx.check_cfg(package='cairo', atleast_version=cairo_version_required,
                  args='--cflags --libs')
 
+  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'])