build: Prepend project common path

This avoids conflicts with standard packages such as PIL (Python Imaging
Library).

user/wscript

@@ -1,6 +1,6 @@
 from sys import path
 from os.path import abspath
-path.append(abspath('../common/'))
+path.insert(0, abspath('../common/'))
 
 from waf import cmd_configure as configure
 from waf import cmd_build as build