gnu: libsigsegv: Fix mips64el-linux-gnu (cross-)builds.

* gnu/packages/libsigsegv.scm (libsigsegv): New `arguments' field.

gnu: findutils: Fix cross-compilation.

* gnu/packages/base.scm (findutils): Reinstate cross-compilation
  #:configure-flags.

gnu: sed: Allow cross-compilation.

* gnu/packages/base.scm (sed): Keep the default phases
  when (%current-target-system) is true.

gnu: coreutils: Allow cross-compilation.

* gnu/packages/base.scm (coreutils): Remove Perl input
  when (%current-target-system) is true.  Use %standard-cross-phases as
  the base when (%current-target-system) is true.

gnu: gettext, attr, acl: Disable `check' phase twiddling when cross building.

* gnu/packages/gettext.scm (gettext)[arguments]: Disable `check' phase
  manipulations when (%current-target-system) is true.
* gnu/packages/attr.scm (attr)[arguments]: Likewise.
* gnu/packages/acl.scm (acl)[arguments]: Likewise.

packages: Make `native-inputs' thunked.

* guix/packages.scm (<package>): Make `native-inputs' thunked.

packages: Re-export `%current-target-system'.

* guix/packages.scm: Re-export `%current-target-system'.

build-system/gnu: Make sure build input variables contain pairs.

* guix/build-system/gnu.scm (gnu-cross-build)[builder]: Make sure
  %build-host-inputs and %build-target-inputs always contain pairs, not
  lists.

build: `hydra.scm' cross-builds simpler packages.

* hydra.scm (%packages-to-cross-build): Remove things from
  make-bootstrap for now.

build-system/trivial: Implement the cross-build protocol.

* guix/build-system/trivial.scm (guile-for-build): New procedure.
  (trivial-build): Use it.
  (trivial-cross-build): New procedure.
  (trivial-build-system): Use it.

package: Fix default profile ownership check when it's just been created.

* guix/scripts/package.scm (guix-package)[ensure-default-profile]: Check
  whether S is true before checking its owner.  Fixes the case where we
  built %PROFILE-DIRECTORY just above.

build: `hydra.scm' adds cross-build jobs.

* hydra.scm (package->alist): Add `package-derivation' argument.
  (package-cross-job): New procedure.
  (%packages-to-cross-build, %cross-targets): New variables.
  (hydra-jobs): Add cross jobs.

build: Add `--target' option.

* guix/scripts/build.scm (derivations-from-package-expressions): Add
  `package-derivation' parameter.
  (show-help, %options): Add `--target'.
  (guix-build): Use `package-cross-derivation' when `--target' is
  passed.
* tests/guix-build.sh: Add dry-run test with `--target'.
* doc/guix.texi (Invoking guix build): Document `--target'.

build-system/gnu: Implement cross build.

* guix/build-system/gnu.scm (inputs-search-paths): New procedure.
  (standard-search-paths): Use it.
  (expand-inputs): New procedure.
  (standard-inputs): Use it.
  (standard-cross-packages, standard-cross-inputs,
  standard-cross-search-paths, gnu-cross-build): New procedures.
  (gnu-build-system): Set `cross-build' field to `gnu-cross-build'.
* gnu/packages/cross-base.scm: Export `cross-gcc', `cross-binutils', and
  `cross-libc'.
* guix/build/gnu-cross-build.scm: New file.
* Makefile.am (MODULES): Add it.

packages: Implement `package-cross-derivation'.

* guix/packages.scm (package-transitive-target-inputs,
  package-transitive-native-inputs): New procedures.
  (package-derivation): Parametrize `%current-target-system'.
  (package-cross-derivation): Implement.
* guix/utils.scm (%current-target-system): New variable.
* tests/packages.scm ("package-cross-derivation"): New test.
* doc/guix.texi (Defining Packages): Document
  `package-cross-derivation'.

gnu: cross-gcc: Set the right search paths.

* gnu/packages/cross-base.scm (cross-gcc): Override `search-paths' and
  `native-search-paths'.

gnu: cross-gcc: Change patch so that `CROSS_LIBRARY_PATH' is honored.

* gnu/packages/patches/gcc-cross-environment-variables.patch: Add two
  hunks, most notably one that changes GCC to honor LIBRARY_PATH_ENV
  when cross-compiling.

packages: Factorize things common to `package-{,cross-}derivation'.

* guix/packages.scm (expand-input): New procedure, moved out of...
  (package-derivation): ... here.  Adjust accordingly.
  (package-cross-derivation): Add `cross-system' and `system'
  parameters.

nls: Add Esperanto translation.

* po/eo.po, po/LINGUAS: New files.

gnu: Add UnRTF.

* gnu/packages/unrtf.scm: New file.
* Makefile.am (MODULES): Add it.