* config-daemon.ac: Don't bail out when libbz2 is missing.  Define
'HAVE_LIBBZ2' Automake conditional.
* nix/libstore/build.cc: Wrap relevant bits in '#if HAVE_BZLIB_H'.
* nix/libstore/globals.cc (Settings::Settings): 'logCompression'
defaults to COMPRESSION_GZIP when HAVE_BZLIB_H is false.
* nix/libstore/globals.hh (CompressionType): Make 'COMPRESSION_BZIP2'
conditional on HAVE_BZLIB_H.
* nix/local.mk (guix_register_LDADD, guix_daemon_LDADD): Add -lbz2 only
when HAVE_LIBBZ2.
* nix/nix-daemon/guix-daemon.cc (parse_opt): Ignore "bzip2" when not
HAVE_BZLIB_H.