build-system/gnu: Add `static-libgcc-package' & supporting procedure.

* guix/build-system/gnu.scm (package-with-extra-configure-variable,
  static-libgcc-package): New procedures.

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'.

build-system/gnu: Pass the system type to the builder.

* guix/build-system/gnu.scm (gnu-build)[builder]: Pass SYSTEM to
  `gnu-build' as a keyword argument.

build-system/{gnu,trivial-build}: Fix handling of #:guile argument.

* guix/build-system/gnu.scm (gnu-build)[guile-for-build]: Check whether
  GUILE matches string? before checking whether it matches
  derivation-path?.
* guix/build-system/trivial.scm (trivial-build)[guile-for-build]:
  Likewise.

build-system/{gnu,trivial}: Add a `#:guile' keyword parameter.

* guix/build-system/gnu.scm (package-with-explicit-inputs): New `guile'
  keyword parameter.  Add it to P's arguments, and pass it in recursive
  calls.
  (gnu-build): New `guile' keyword parameter; new `guile-for-build'
  variable.  Pass it as the `#:guile-for-build' parameter of
  `build-expression->derivation'.

* guix/build-system/trivial.scm (trivial-build): Likewise.

distro: Rename (distro ...) to (distro packages ...).

* distro/base.scm, distro/ld-wrapper.scm: Move to `distro/packages'.
  Adjust LD-WRAPPER-BOOT3 input file name accordingly.
* Makefile.am (MODULES): Adjust accordingly.
* distro.scm (%distro-module-directory): Change to "/distro/packages".
* guix/build-system/gnu.scm (standard-inputs): Change module name
  to (distro packages base).
* tests/packages.scm (test-packages): Likewise.

build-system/gnu: Relax location handling in `package-with-explicit-inputs'.

* guix/build-system/gnu.scm (package-with-explicit-inputs): Convert LOC
  when it is a source-property list.

distro: Bootstrap standard inputs from Nixpkgs.

This is a first step towards bootstrapping from a set of pre-built,
statically-linked binaries.

* guix/build-system/gnu.scm (package-with-explicit-inputs,
  standard-inputs): New procedure.
  (%store): New variable.
  (%standard-inputs): Remove.
  (gnu-build): New `implicit-inputs?' keyword parameter.  Use it to
  choose whether to use `(standard-inputs SYSTEM)' or the empty list.

* distro/base.scm (guile-2.0): Remove dependency on XZ, which is now
  implicit.
  (%bootstrap-inputs, gcc-boot0, binutils-boot0, linux-headers-boot0,
  %boot1-inputs, glibc-final, %boot2-inputs, m4-boot2, gmp-boot2,
  mpfr-boot2, mpc-boot2, %boot3-inputs, gcc-final, %boot4-inputs,
  %final-inputs): New variables.

build-system/gnu: Add `path-exclusions' parameter.

* guix/build/gnu-build-system.scm (set-paths): Add new `path-exclusions'
  parameter; honor it.
* guix/build-system/gnu.scm (gnu-build): New `path-exclusions' keyword
  parameter; pass it to BUILDER.

* distro/base.scm (gcc-4.7): Exclude "libc" from $LIBRARY_PATH.

build-system/gnu: Add a `strip' phase.

* guix/build/gnu-build-system.scm (strip): New procedure.
  (%standard-phases): Add it.

* guix/build-system/gnu.scm (gnu-build): New `strip-binaries?',
  `strip-flags', and `strip-directories' keyword parameters.  Pass them
  to BUILDER.

build-system/gnu: Add `out-of-source?' keyword parameter.

* guix/build/gnu-build-system.scm (configure): Add an `out-of-source?'
  keyword parameter; build out-of-source-tree when #t.
* guix/build-system/gnu.scm (gnu-build): Add `out-of-source?' keyword
  parameter.  Pass it in BUILDER.

build-system/gnu: Add a `patch-shebangs' phase.

* guix/build/gnu-build-system.scm (patch-shebangs): New procedure.
  (%standard-phases): Add it.

* guix/build-system/gnu.scm (gnu-build): New `patch-shebangs?' keyword
  parameter.  Pass it to the builder's `gnu-build'.

build-system/gnu: Add dependency on GNU Findutils.

* guix/build-system/gnu.scm (%standard-inputs): Add "findutils".

build-system/gnu: Add `tests?' parameter.

* guix/build-system/gnu.scm (gnu-build): Add `tests?' parameter.
  [builder]: Inherit it.

build-system/gnu: Support parallel builds and tests.

* guix/build/gnu-build-system.scm (build): Add `parallel-build?'
  parameter; honor it and $NIX_BUILD_CORES.
  (check): Add `parallel-tests?' parameter; likewise.

* guix/build-system/gnu.scm (gnu-build): Add `parallel-build?' and
  `parallel-tests?' parameters.
  [builder]: Inherit them.

build-system/gnu: Add a `patch' phase.

* guix/build/gnu-build-system.scm (patch): New procedure.
  (%standard-phases): Add `patch'.

* guix/build-system/gnu.scm (gnu-build): Add `patches' and `patch-flags'
  parameters.  Pass them on.

build-system/gnu: Make the builder's module list a parameter.

* guix/build-system/gnu.scm (gnu-build): Add a `modules' keyword
  parameter; use it.

build-system/gnu: Add GNU Awk to the standard inputs.

* guix/build-system/gnu.scm (%standard-inputs): Add GNU Awk.

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.

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.