added DONT_PATCH_SETUPTOOLS environment marker

--HG--
branch : distribute
extra : rebase_source : 4fe53262211c90f2a90027014970f7e666de88f0

setup.py

@@ -75,6 +75,8 @@ def _buildout_marker():
         return 'buildout' in os.path.basename(command)
 
 def _being_installed():
+    if os.environ.get('DONT_PATCH_SETUPTOOLS') is not None:
+        return True
     if _buildout_marker():
         # Installed by buildout, don't mess with a global setuptools.
         return False