build-system/gnu: Have `configure-flags' and `make-flags' evaluated.

* guix/build-system/gnu.scm (gnu-build): Double-quote the default value
  of MAKE-FLAGS and CONFIGURE-FLAGS.  Don't quote them in BUILDER.

build: Define $NIXPKGS when compiling.

* Makefile.am (.scm.go): Define $NIXPKGS.

Abstract build systems.

* Makefile.am (MODULES): Add `guix/build-system.scm' and
  `guix/build-system/gnu.scm'.  Remove `guix/gnu-build-system.scm'.

* guix/build-system.scm: New file.
* guix/gnu-build-system.scm: Rename to...
* guix/build-system/gnu.scm: ... this.
  (gnu-build-system): New variable.

* tests/builders.scm: Adjust `use-module' clauses.
  ("gnu-build-system"): New test.

Add `define-record-type*'.

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

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

build-expression->derivation: Move module forms at the top-level.

* guix/derivations.scm (build-expression->derivation)[module-form?]: New
  procedure.
  [prologue]: Add any form of EXP that matches MODULE-FORM?.
  [builder]: Remove any MODULE-FORM? from EXP.

gnu-build-system: Structure as a customizable sequence of phases.

* guix/build/gnu-build-system.scm (set-paths, build, check, install):
  New procedures.
  (unpack): Make `source' a keyword arg; add `#:allow-other-keys'.
  (configure): Likewise.
  (%standard-phases): New variable.
  (gnu-build): Make `source', `outputs', and `inputs' keyword arguments;
  add `phases' keyword argument; #:allow-other-keys; add rest arguments
  `args'.  Invoke each of PHASES in order within `every'.

* guix/gnu-build-system.scm (gnu-build): Add `make-flags' and `phases'
  keyword arguments.  Update builder's `gnu-build' call to match the new
  convention.

gnu-build-system: Add GNU Diffutils and GNU Path to the standard inputs.

* guix/gnu-build-system.scm (%standard-inputs): Add Diffutils and Patch.

build-expression->derivation: Have the build fail when EXP returns #f.

* guix/derivations.scm (build-expression->derivation)[builder]: Pass the
  result of EXP to `exit'.

* tests/derivations.scm ("build-expression->derivation with expression
  returning #f"): New test.

Remove leftover debugging expressions.

* guix/build/utils.scm (directory-exists?): Remove leftover debugging
  expressions.

store: Wait for the server to be done sending output.

* guix/store.scm (current-build-output-port): New variable.
  (process-stderr): Add docstring.  Always return #f, except upon
  %STDERR-LAST.  Upon %STDERR-NEXT, write to
  `current-build-output-port', not `current-error-port'.
  (set-build-options): Loop until `process-stderr' returns true.
  (define-operation): Likewise.
  (build-derivations): Update docstring to mention that it's
  synchronous.

build: Be cross-compilation-ready.

* Makefile.am (.scm.go): Pass `--target=$(host)'.

* configure.ac: Use `AC_CANONICAL_HOST'.

build: Provide the right include path when building modules.

* Makefile.am (.scm.go): Add missing `-L $(top_srcdir)'.

Build (guix) last.

* Makefile.am (MODULES): Move `guix.scm' last.

Add a build system.

* configure.ac, Makefile.am: New files.

Change `build-expression->derivation' to support sub-derivation inputs.

* guix/derivations.scm (build-expression->derivation): Change to expect
  INPUTS to have the form (NAME DRV-PATH SUB-DRV) or (NAME DRV-PATH),
  instead of (NAME . DRV-PATH).  Update callers accordingly.

* guix/gnu-build-system.scm, tests/builders.scm, tests/derivations.scm:
  Update accordingly.

Add supporting tools for the GNU Build System.

* guix/derivations.scm (build-expression->derivation): Add all of INPUTS
  as inputs to the final derivation.

* guix/build/gnu-build-system.scm, guix/build/utils.scm,
  guix/gnu-build-system.scm: New files.

* tests/builders.scm ("gnu-build"): New test.

Fix the order of the derivation's sources in `derivation-hash'.

* guix/derivations.scm (derivation-hash): Sort SOURCES alphabetically as
  well.

Fix `imported-files' for files with a common prefix.

* guix/derivations.scm (imported-files)[builder]: For TAIL, check
  whether it exists before (mkdir TAIL).

* tests/derivations.scm ("imported-files"): Add file with a common
  prefix.

Add `add-to-store' test for flat files.

* tests/derivations.scm ("add-to-store, flat"): New test.

Augment `build-expression->derivation' with #:modules; add `http-fetch'.

* guix/derivations.scm (imported-modules): New procedure.
  (build-expression->derivation): New keyword argument `modules'.
  Use `imported-modules' when MODULES is non-empty, and pass it with
  `-L' to GUILE.

* guix/build/http.scm, guix/http.scm, tests/builders.scm: New files.