* guix/scripts/build.scm (%options, show-help): Add `--fallback'.
  (guix-build): Call `set-build-options' with #:fallback?.
* guix/scripts/package.scm (%options, show-help): Add `--fallback'.
  (guix-package): Call `set-build-options' with #:fallback?.
* doc/guix.texi (Invoking guix package, Invoking guix build): Document
  `--fallback'.

* guix/store.scm (set-build-options): Rename #:try-fallback? to #:fallback?.
* tests/store.scm ("substitute --fallback"): New test.

* gnu/packages/python.scm (pytz): New package.

* gnu/packages/tor.scm (torsocks): New variable.

* guix/build/python-build-system.scm (check): New procedure.
  (%standard-phases): Use it.

* guix/packages.scm (&package-cross-build-system-error): New condition type.
  (package-cross-derivation): Raise &package-cross-build-system-error
  when the build system doesn't support cross builds.
* guix/ui.scm (call-with-error-handling): Add
  package-cross-build-system-error? case.
* tests/packages.scm ("package-cross-derivation, no cross builder"): New test.

* guix/build-system/gnu.scm (gnu-cross-build)[builder]: When
  IMPLICIT-TARGET-INPUTS is #f, default to '().

* gnu/packages/guile.scm (guile-2.0): When cross-compiling, unset
  GUILE_LOAD_PATH and GUILE_LOAD_COMPILED_PATH, and set
  CC_FOR_BUILD=gcc.

* gnu/packages/pkg-config.scm (pkg-config): Rename to...
  (%pkg-config): ... this.  Make private.
  (cross-pkg-config, pkg-config-for-target): New procedures.
  (pkg-config): New macro.

This fixes resolution of DT_NEEDED entries in cross-built libraries.
For instance, if ltdl.so needs libdl.so and has it in its RUNPATH, then
libdl.so is searched for in the right place.

* gnu/packages/cross-base.scm (cross-binutils): Pass
  `--with-sysroot=/no-such-path'.

* doc/guix.texi (Introduction, Requirements): Rephrase and fix typos.

* gnu/packages/guile.scm (guile-2.0): Add Bash as an input
  when (%current-target-system) is true.  Use %standard-cross-phases as
  a basis when (%current-target-system) is true.

* gnu/packages/bash.scm (bash): Pass "bash_cv_job_control_missing=no"
  when (%current-target-system) is true.

* hydra.scm: Import (gnu packages gettext).

This is especially useful since the tarball is deleted from zlib.net as soon as
a new version of zlib is released.

* hydra.scm: Import (gnu packages gawk).

* hydra.scm (%packages-to-cross-build): Add a few packages.

* gnu/packages/ncurses.scm (ncurses): Add `cross-pre-install-phase'.
  Use different phases when (%current-target-system) is true.
* gnu/packages/readline.scm (readline): Use %standard-cross-phases as
  the base when (%current-target-system) is true.

* gnu/packages/libffi.scm (libffi): Use %standard-cross-phases as the
  base when cross-compiling.  Add (guix build gnu-cross-build) to the
  #:modules when cross-compiling.  Improve synopsis.

* gnu/packages/autotools.scm (libtool)[arguments]: Use #:phases argument
  only when building natively.

* gnu/packages/gawk.scm (gawk)[arguments]: Use `set-shell-file-name'
  phase that works when cross-compiling; use `%standard-cross-phases' as
  the base when (%current-target-system) is true.

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

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

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

* 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/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.

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

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

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

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

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

* 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.

* 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.

* 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'.

* 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.

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

* 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.

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

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