build: Produce (guix config) instead of using compile-time tricks.

* guix/config.scm.in: New file.
* guix/utils.scm: Use it.
  (%libgcrypt): Remove.
  (%nixpkgs-directory): Don't capture the compile-time $NIXPKGS; use
  %NIXPKGS instead.
  (nixpkgs-derivation): Use %NIX-INSTANTIATE.
* pre-inst-env.in (NIX_INSTANTIATE, NIXPKGS, LIBGCRYPT): Remove.

* configure.ac: Emit `guix/config.scm'.
* Makefile.am (GOBJECTS): Add `guix/config.go'.
  (nobase_nodist_guilemodule_DATA): Add `guix/config.scm'.

build: Clearly mark Nixpkgs as optional.

* configure.ac: Always show the result of checking for Nixpkgs.  Don't
  warn when Nixpkgs is not found.
* Makefile.am (AM_DISTCHECK_CONFIGURE_FLAGS): Remove `--with-nixpkgs'
  flag.
* guix/utils.scm (%nixpkgs-directory): Use either the compile-time or
  the run-time `NIXPKGS' environment variable.
* release.nix (jobs.tarball, jobs.build): Remove `--with-nixpkgs'
  configure flag.
* README: Mark Nixpkgs as optional.

* distro/packages/databases.scm, distro/packages/guile.scm,
  distro/packages/typesetting.scm: Change uses of `nixpkgs-derivation*'
  to `nixpkgs-derivation', to avoid failing at compile-time.

tests: Run without substitutes.

* tests/builders.scm, tests/derivations.scm, tests/packages.scm: Set
  #:use-substitutes? #f for %STORE.

Add a preliminary `guix-package' command-line tool.

* guix-package.in, tests/guix-package.sh: New files.
* configure.ac: Output `guix-package'.
* Makefile.am (TESTS): Add `tests/guix-package.sh'.
  (bin_SCRIPTS): Add `guix-package'.

store: Add `store-path-package-name'.

* guix/store.scm (store-path-package-name): New procedure.
* tests/utils.scm ("store-path-package-name"): New test.

utils: Remove stuff committed by error.

This was wrongfully committed in df1fab58.

* guix/utils.scm (hash-set-proc, hash-ref*, memoizing-lambda): Remove.

build: Add unit test for the `guix-build' command.

* tests/guix-build.sh: New file.
* Makefile.am (TESTS): Add it.
  (LOG_COMPILER): Rename to...
  (SCM_LOG_COMPILER): ... this.  Move flags to...
  (AM_SCM_LOG_FLAGS): ... this.
  (TEST_EXTENSIONS, SH_LOG_COMPILER, AM_SH_LOG_FLAGS): New variables.

Add (guix ui).

* guix/ui.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Add it.

* guix-build.in: Use it.
  (_, N_, leave): Remove.
  (guix-build): Use `with-error-handling' instead of the `guard' form.
* guix-download.in: Use it.
  (_, N_, leave): Remove.

build-system/gnu: Distinguish between imported modules and used modules.

* guix/build-system/gnu.scm (gnu-build): Add the `imported-modules'
  keyword parameter.  Pass it to `build-expression->derivation'.

release.nix: Use `--no-substitutes' in `distro.hello'.

* release.nix (distro.hello): Use `guix-build --no-substitutes', to
  avoid failures due to unavailable stale substitutes on
  hydra.nixos.org.

guix-build: Add `--root'.

* guix/store.scm (add-indirect-root): New operation.
* guix-build.in (show-help): Document `--root'.
  (%options): Add `--root'.
  (guix-build)[register-root]: New procedure.  Call it when `--root' is
  passed.

Add (guix build union).

* guix/build/union.scm, tests/union.scm: New files.
* Makefile.am (MODULES): Add `guix/build/union.scm'.
  (TESTS): Add `tests/union.scm'.

release.nix: Change `distro.hello' to produce something.

* release.nix (distro.hello)[buildPhase]: Tee the log to $out.  Add a
  `name' attribute; remove `buildInputs' and instead use the full path
  to `guix-build'.

distro: Build Bash sequentially.

* distro/packages/base.scm (bash): Set `#:parallel-build?' and
  `#:parallel-tests?' to #f.

tests: Use our own bootstrap tools.

* tests/builders.scm (%bootstrap-inputs): Use %BOOT0-INPUTS from the distro.
* tests/packages.scm (%bootstrap-inputs): Likewise.
* tests/derivations.scm (%coreutils): Alias for %BOOTSTRAP-COREUTILS&CO.

release.nix: Add a `distro.hello' job.

* release.nix (distro.hello): New job.

build: Preserve the executable bit of bootstrap binaries.

* Makefile.am (install-data-hook): New target.

build: Make sure scripts know where to find their modules.

* guix-build.in, guix-download.in: Define `prefix' and `datarootdir', so
  that `guilemoduledir' expands to something meaningful.

release.nix: Pre-download the Guile bootstrap tarball.

* release.nix (bootstrap_guile): New variable.
  (build)[preBuild]: New attribute.

build: Fix out-of-source-tree builds.

* Makefile.am (distro/packages/bootstrap/x86_64-linux/guile-bootstrap-2.0.6.tar.xz,
  distro/packages/bootstrap/i686-linux/guile-bootstrap-2.0.6.tar.xz):
  Make the target's parent directory.
* pre-inst-env.in (DISTRO_BOOTSTRAP_PATH): Add the builddir-relative
  directory.