doc: Improve wording and fix typos in "Introduction" and "Requirements".

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

gnu: guile: Add Bash as an input, to permit cross-compilation.

* 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: bash: Fix cross-compilation.

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

build: `hydra.scm' really cross-builds Gettext.

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

zlib: Add a sourceforge mirror.

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

build: Add missing import in `hydra.scm'.

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

build: `hydra.scm' cross-builds a few more packages.

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

gnu: ncurses, readline: Allow cross-compilation.

* 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: libffi: Allow cross-compilation.

* 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: libtool: Allow cross-compilation.

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

gnu: gawk: Allow cross-compilation.

* 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: 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.