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.

utils: Remove stuff committed by error.

This was wrongfully committed in df1fab58.

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

location: Start column numbers at 1.

* guix/utils.scm (source-properties->location): Use COL + 1.

Add a `system' parameter to `nixpkgs-derivation'.

* guix/utils.scm (nixpkgs-derivation): Add a `system' parameter.  Pass
  it in the `nix-instantiate' invocation.

distro: Add a statically-linked, relocatable Guile 2.0 package.

* distro/packages/base.scm (%guile-static, %guile-static-stripped): New
  variables.

* distro/patches/guile-relocatable.patch: New file.
* Makefile.am (dist_patch_DATA): Add it.

Move <location> to (guix utils).

* guix/packages.scm (<location>, location): Move to...
* guix/utils.scm: ... here.

define-record-type*: Add the `inherit' syntactic constructor keyword.

* guix/utils.scm (define-record-type*)[make-syntactic-constructor]: New
  `type' parameter.  Add the `inherit' keyword and corresponding support
  code.

* tests/utils.scm ("define-record-type* & inherit", "define-record-type*
  & inherit & letrec* behavior"): New tests.

build: Add `--with-libgcrypt-prefix'; capture the path to libgcrypt.

* configure.ac: Add `--with-libgcrypt-prefix'.  Substitute `LIBGCRYPT'.

* Makefile.am (.scm.go): Pass $(LIBGCRYPT).

* guix/utils.scm (%libgcrypt): New variable.
  (sha256): Use it.

build: Pass $(NIX_INSTANTIATE) to the build and test environments.

* Makefile.am (.scm.go): Define $NIX_INSTANTIATE.
  (TESTS_ENVIRONMENT): Likewise, and define $NIX_HASH.
* guix/utils.scm (nixpkgs-derivation): Use $NIX_INSTANTIATE when
  defined.

* tests/utils.scm (%nix-hash): New variable.
  ("sha256 & bytevector->nix-base32-string"): Use it.

Add `nixpkgs-derivation*'; use it in the distro.

* guix/utils.scm (nixpkgs-derivation*): New macro.

* distro/base.scm: Use it instead of `nixpkgs-derivation'.

define-record-type*: Add `letrec*' behavior.

* guix/utils.scm (define-record-type*)[make-syntactic-constructor]: Bind
  all the ((FIELD VALUE) ...) in a `letrec*'.  Adjust `field-value'
  accordingly.

* tests/utils.scm ("define-record-type* with letrec* behavior"): New
  test.

Make `define-record-type*' error messages more informative.

* guix/utils.scm (define-record-type*): In case of missing or extra
  field initializers, raise a descriptive `syntax-error'.

Add a libgcrypt-based implementation of `sha256'.

* guix/utils.scm (sha256): Add a libgcrypt-based implementation using
  the FFI.

Add a sha256 fallback that uses Coreutils instead of libchop.

* guix/utils.scm (compile-time-value): Move to the top.
  (sha256): Add an implementation that uses Coreutils, for when libchop
  is unavailable.

Introduce `compile-time-value' and use it.

* guix/utils.scm (compile-time-value): New macro.
  (%nixpkgs-directory): Use it.

* guix/build-system/gnu.scm (%standard-inputs): Likewise.

Add `define-record-type*'.

* guix/utils.scm (define-record-type*): New macro.

* tests/utils.scm ("define-record-type*"): New test.

Add a `%current-system' fluid.

* guix/utils.scm (gnu-triplet->nix-system): New procedure.
  (%current-system): New variable.

* tests/utils.scm ("gnu-triplet->nix-system"): New test.

* tests/derivations.scm (%current-system): Remove.  Update users to
  use (%current-system) instead.

Add `base16-string->bytevector'.

* guix/utils.scm (base16-string->bytevector): New procedure.

* tests/utils.scm ("bytevector->base16-string->bytevector"): New test.

Add `base32-string->bytevector' and `nix-base32-string->bytevector'.

* guix/utils.scm (bytevector-quintet-set!,
  bytevector-quintet-set-right!, base32-string-unfold,
  base32-string-unfold-right, make-base32-string->bytevector,
  base32-string->bytevector, nix-base32-string->bytevector): New
  procedures.

* tests/utils.scm ("base32-string->bytevector",
  "nix-base32-string->bytevector"): New tests.

Allow derivations with input derivations.

* guix/derivations.scm (derivation-path->output-path): New procedure.
  (derivation-hash): Call `memoize'.  In the fixed-output case, convert
  HASH-ALGO to a string.  In the other case, sort inputs in the
  alphabetical order of their hex hash.  For inputs with no sub-drvs,
  add "out" as the sub-drv.

* guix/utils.scm (%nixpkgs-directory): New parameter.
  (nixpkgs-derivation, memoize): New procedures.

* tests/derivations.scm ("build derivation with 1 source"): Remove
  useless shebang.
  (%coreutils): New variable.
  ("build derivation with coreutils"): New test.