python-build-system: add a check phase.

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

packages: Raise an error condition a cross builder is needed but unavailable.

* 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.

build-system/gnu: Fix typo in `gnu-cross-build'.

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

gnu: guile: More cross-compilation tweaks.

* 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: pkg-config: Provide a cross-build-friendly wrapper.

* 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.

gnu: cross-binutils: Build with `--with-sysroot'.

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: 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.