compression: add separate pre-processor definitions

compression: add separate pre-processor definitions

Follow-up for https://github.com/systemd/systemd/commit/cd3c6322dbc6370448bafc216ee4e19e32a79d9e

journal-def.h should be self-contained too, as it represents the journal object ABI.
Duplicate the enums, as they also need to be in config.h for it to be self-contained,
and enums are not available to the preprocessor. Use an assert to ensure they don't
diverge.