rules: Disable preprocessor warnings on kfreebsd.

This is to work around an error resulting from a redefined macro in
PostgreSQL's pg_config_manual.h.

Thanks to Ivo De Decker for reporting this.
Closes: #760719
Build-Depend on libprotobuf-c-dev instead of libprotobuf-c0-dev.

This is due to the protobuf-c transition.
Closes: #756422
Merged branch 'nmu5.4.1-3.1'.
Disable the 'java' plugin on sparc for now. It fails to build.
Released 5.4.1-4.