Propagate sys.path as PYTHONPATH while develop

  While invoke setup.py certain eggs (like scikit-learn) launch
  cetain custom builds (for cython) using subprocess and sys.executable.

  This commit aims to preserve the sys.path over the runs, even if an
  egg is using subprocess with the same python to build a component of
  the egg.

src/zc/buildout/easy_install.py

@@ -1621,6 +1621,8 @@ for extra_path in %(path_list)r:
 
 import os, setuptools
 
+os.environ['PYTHONPATH'] = (os.pathsep).join(sys.path[:])
+
 __file__ = %(__file__)r
 
 os.chdir(%(setupdir)r)