When buildout generates scripts, it injects the following prologue for
them:

   #!/path/to/underlying/python

   import sys
   sys.path[0:0] = [
     'path/to/egg1',
     'path/to/egg2',
     ...
   ]

   import SCRIPT
   SCRIPT.main()

This way path to eggs are inserted before what was originally
sys.path[0] and it breaks gpython check that `sys.path[0] == exedir`.

-> Fix it by detecting buildout and in that case verifying that exedir
is only present in sys.path, not located in sys.path[0].