message: [lit] Fix the way we check if an environment var is set
  The old method would throw a KeyError.
diff: @@ -39,7 +39,7 @@ def find_shlibpath_var():
  elif platform.system() == 'Windows':
  yield 'PATH'
  -if not config.environment['DYLD_INSERT_LIBRARIES']:
  +if 'DYLD_INSERT_LIBRARIES' in os.environ:
  # Shared library build of LLVM may require LD_LIBRARY_PATH or equivalent.
  # This clashes with DYLD_INSERT_LIBRARIES which is needed on Darwin.
  for shlibpath_var in find_shlibpath_var():
  