config_setting() handling and support for flags.

This release adds support for config_setting() and associated flags.
These can be listed with list-flags and list-settings.

The configuration settings are also properly evaluated in select(). Also, bant evaluated --//foo and --@foo// options and handles them similar to how bazel would do that (note: there was rudimentary flag support before, but that only directly selected the config settings; so now the behavior is more accurate).

Also:

• fix some macro expansion issue.
• Expand reading .bazelignore also in all modules we depend on to not pick up unrelated BUILD files in these (e.g. some projects have a vendored googletest in a subdirectory for use with other build tools but then filter them out with .bazelignore