LIT uses a model where the test suite is configurable trough a lit.site.cfg file. Most of the time we use the lit.site.cfg which has a bunch of default values, generated by CMake, that match the current build configuration. However, nothing prevents you from running the test suite with a different configuration, either by overriding some of these values from the command line, or by passing a different lit.site.cfg.
Many of those configurationvalues are optional but they still need to be set because lit.cfg.py is accessing them directly. This patch changes the code to use getattr to return the attribute if it exists. This makes it possible to specify a minimal lit.site.cfg with only the mandatory/desired configuration values.