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'.

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.

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.

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.

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.

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.

gnu: Add UnRTF.

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

gnu: libgc: Add 7.2d.

* gnu/packages/bdw-gc.scm (libgc-7.2): New variable.