* configure.ac: Compute and substitute `guix_localstatedir'.
* m4/guix.m4: Substitute `guix_system'.
* guix/config.scm.in (%store-directory, %store-directory, %system): New
variables.
* guix/store.scm (%nix-state-dir): Remove.
(%default-socket-path): Use %STATE-DIRECTORY as the default.
(%store-prefix): Use %STORE-DIRECTORY as the default.
* guix/utils.scm (%current-system): Default to %SYSTEM.