Rename `BUILD_ATLAS` option
This should be USE_EXTERNAL_ACTS and be off by default. Clarifies that this is not the default configuration. All options are then off by default, which is consistent with the options in the core.
This should be USE_EXTERNAL_ACTS and be off by default. Clarifies that this is not the default configuration. All options are then off by default, which is consistent with the options in the core.