build: Include a copy of Nix's libstore and daemon; build it.
* configure.ac: Call `AC_USE_SYSTEM_EXTENSIONS', and
`GUIX_SYSTEM_TYPE'. Add `--with-store-dir' option, and substitute
`storedir'. Include `config-daemon.ac'.
* config-daemon.ac: New file.
* Makefile.am [BUILD_DAEMON]: Include `daemon.am'.
* daemon.am: New file.
* m4/guix.m4 (GUIX_SYSTEM_TYPE): New macro.
* nix/libutil/gcrypt-hash.cc, nix/libutil/gcrypt-hash.hh,
nix/libutil/md5.h, nix/libutil/sha1.h, nix/libutil/sha256.h,
nix/nix-daemon/guix-daemon.cc, nix/nix-daemon/shared.hh: New files.
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