lit/Util.py: On Cygwin, 'PATHEXT' may exist but it should not be used.
lit/Util.py: On Cygwin, 'PATHEXT' may exist but it should not be used.

git-svn-id: a4a6f32337ebd29ad4763b423022f00f68d1c7b7@125272 91177308-0d34-0410-b5e6-96231b3b80d8

lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx