gnu: Read 'C_INCLUDE_PATH' instead of 'CPATH'.

This is a followup to 009b53fd.

* gnu/packages/cmake.scm (cmake)[arguments]: Use (getenv
"C_INCLUDE_PATH") instead of (getenv "CPATH").
* gnu/packages/package-management.scm (rpm)[arguments]: Likewise.
* gnu/packages/video.scm (avidemux)[arguments]: Likewise.
* gnu/packages/webkit.scm (webkitgtk)[arguments]: Likewise.
* gnu/packages/cross-base.scm (cross-gcc-arguments): Likewise, and unset
'C_INCLUDE_PATH' and 'CPLUS_INCLUDE_PATH'.

gnu: python: Honor 'C_INCLUDE_PATH'.

This is a followup to 009b53fd.

* gnu/packages/patches/python-3-search-paths.patch: Check
'C_INCLUDE_PATH' instead of 'CPATH'.
* gnu/packages/patches/python-2.7-search-paths.patch: Likewise.

gnu: libgcrypt: Update to 1.6.4.

* gnu/packages/gnupg.scm (libgcrypt): Update to 1.6.4.

gnu: libgpg-error: Update to 1.21.

* gnu/packages/gnupg.scm (libgpg-error): Update to 1.21.

gnu: tzdata: Update to 2015g.

* gnu/packages/base.scm (tzdata): Update to 2015g.

Merge branch 'master' into core-updates

gnu: grep: Skip timing-sensitive test.

This avoids failures on loaded machines as seen
at <http://hydra.gnu.org/build/906900/log/raw>.

* gnu/packages/patches/grep-timing-sensitive-test.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/base.scm (grep): Use it.

gnu: pkg-config: Update to 0.29.

* gnu/packages/pkg-config.scm (%pkg-config): Update to 0.29.

gnu: libgc: Enable C++ support.

* gnu/packages/bdw-gc.scm (libgc-7.2, libgc)[arguments]: Add
  --enable-cplusplus to #:configure-flags.

graph: %BAG-WITH-ORIGINS-NODE-TYPE includes the origin's guile.

Before that it would include #f for most origins since that the default
value of 'origin-patch-guile'.

* guix/scripts/graph.scm (bag-node-edges): When 'origin-patch-guile'
returns #f, use (default-guile).
* tests/graph.scm ("bag DAG, including origins"): Check for an edge from
O to (default-guile).

graph: %BAG-EMERGED-NODE-TYPE filters out origins.

Fixes <http://bugs.gnu.org/22280>.
Reported by Leo Famulari <leo@famulari.name>.

* guix/scripts/graph.scm (%bag-emerged-node-type)[edges]: Mimic
%BAG-NODE-TYPE.  This is a followup to 38b92da.

Update NEWS.

* NEWS: Mention Emacs interface for Hydra and changes in variables.

emacs: Use "B" key in "Package List" to display latest builds.

* emacs/guix-ui-package.el (guix-package-list-latest-builds): New
  procedure.
  (guix-package-list-mode-map): Bind "B" key to it.
  (guix-output-list-mode-map): Likewise.
* doc/emacs.texi (Emacs List buffer): Document it.

emacs: Add Hydra interface.

* emacs/guix-utils.el (guix-hexify, guix-number->bool): New procedures.
  (guix-while-null): New macro.
* emacs/guix-hydra.el: New file.
* emacs/guix-hydra-build.el: New file.
* emacs/guix-hydra-jobset.el: New file.
* emacs.am (ELFILES): Add them.
* doc/emacs.texi (Emacs Hydra): New node.
  (Emacs Interface): Add it.
* doc/guix.texi (Top): Add it.
  (Substitutes): Mention Emacs interface.

emacs: Add 'guix-ui-read-profile'.

* emacs/guix-ui.el (guix-ui-read-profile): New procedure.
* emacs/guix-ui-package.el (guix-search-by-name)
  (guix-search-by-regexp, guix-installed-packages)
  (guix-obsolete-packages, guix-all-available-packages)
  (guix-newest-available-packages): Use it.
* emacs/guix-ui-generation.el (guix-generations)
  (guix-last-generations, guix-generations-by-time): Use it.

emacs: Add hierarchy of customization groups.

* emacs/guix-buffer.el (guix-define-groups, guix-define-entry-type)
  (guix-define-buffer-type): New macros.
  (guix-buffer-define-interface): Add parent groups for the generated
  custom groups.
* emacs/guix-info.el: Use 'guix-define-buffer-type' to generate custom
  groups.
* emacs/guix-list.el: Likewise.
* emacs/guix-ui.el: Use 'guix-define-groups' to generate custom groups.
  (guix-ui-define-entry-type): New macro.
* emacs/guix-ui-package.el: Use it.
* emacs/guix-ui-generation.el: Use it.

emacs: Return nil instead of "" in completing readers.

* emacs/guix-utils.el (guix-completing-read): New procedure.
  (guix-define-readers): Use it.

emacs: Add 'guix-list-get-display-entries'.

* emacs/guix-list.el (guix-list-get-display-entries): New procedure.
* emacs/guix-ui-generation.el (guix-generation-get-display): Use it.

emacs: Add 'list-single' variables.

* emacs/guix-list.el (guix-list-single-entry?): New procedure.
  (guix-list-define-interface): Add ':list-single?' key and generate
  'guix-ENTRY-TYPE-list-single' variables.
* emacs/guix-ui-package.el (guix-package-list-single): Remove.

emacs: Reorganize package/generation UI code.

Move the code for packages/generations interface from "guix-info.el",
"guix-list.el", "guix-base.el" and "guix.el" to "guix-ui-package.el" and
"guix-ui-generation.el".

* emacs/guix-base.el (guix-package-entry->name-specification)
  (guix-package-entries->name-specifications)
  (guix-package-id-and-output-by-output-id)
  (guix-package-installed-outputs, guix-process-package-actions)
  (guix-package-list-type, guix-package-info-type)
  (guix-continue-package-operation-p, guix-get-package-strings)
  (guix-insert-package-strings): Move to "guix-ui-package.el".
  (guix-generation-packages-buffer-name-function, guix-output-name-width)
  (guix-generation-packages-update-buffer, guix-generation-packages)
  (guix-generation-packages-buffer-name-default)
  (guix-generation-packages-buffer-name-long)
  (guix-generation-packages-buffer-name, guix-generation-packages-buffer)
  (guix-generation-insert-packages, guix-generation-insert-package)
  (guix-profile-generation-manifest-file, guix-delete-generations)
  (guix-profile-generation-packages-buffer, guix-switch-to-generation):
  Move to "guix-ui-generation.el".
* emacs/guix-info.el (guix-package-location, guix-package-name)
  (guix-package-source, guix-package-info-source)
  (guix-package-info-heading, guix-package-info-license)
  (guix-package-info-name, guix-package-info-name-button)
  (guix-package-info-version, guix-package-info-location)
  (guix-package-info-synopsis, guix-package-info-description)
  (guix-package-info-obsolete, guix-package-info-installed-outputs)
  (guix-package-info-uninstalled-outputs)
  (guix-package-info-insert-heading)
  (guix-package-info-define-insert-inputs)
  (guix-package-info-obsolete-string)
  (guix-package-info-insert-obsolete-text)
  (guix-package-info-insert-non-unique-text)
  (guix-package-info-insert-outputs, guix-package-info-insert-output)
  (guix-package-info-insert-action-button)
  (guix-package-info-auto-find-source)
  (guix-package-info-auto-download-source)
  (guix-package-info-download-buffer, guix-package-info-show-source)
  (guix-package-info-download-source, guix-package-info-insert-source)
  (guix-package-info-redisplay-after-download)
  (guix-output-info-insert-version, guix-output-info-insert-output):
  Move to "guix-ui-package.el".
  (guix-generation-info-number, guix-generation-info-current)
  (guix-generation-not-current, guix-generation-info-insert-number)
  (guix-generation-info-insert-current): Move to "guix-ui-generation.el".
* emacs/guix-list.el (guix-package-list-generation-marking-enabled)
  (guix-package-list-installed, guix-package-list-obsolete)
  (guix-package-list-get-name, guix-package-list-get-installed-outputs)
  (guix-package-list-marking-check, guix-package-list-mark-outputs)
  (guix-package-list-mark-install, guix-package-list-mark-delete)
  (guix-package-list-mark-upgrade, guix-package-list-mark-upgrades)
  (guix-list-mark-package-upgrades, guix-list-execute-package-actions)
  (guix-package-list-execute, guix-package-list-make-action)
  (guix-package-list-edit, guix-output-list-mark-install)
  (guix-output-list-mark-delete, guix-output-list-mark-upgrade)
  (guix-output-list-mark-upgrades, guix-output-list-make-action)
  (guix-output-list-describe, guix-output-list-edit): Move to
  "guix-ui-package.el".
  (guix-generation-list-get-current, guix-generation-list-switch)
  (guix-generation-list-generations-to-compare)
  (guix-generation-list-compare, guix-generation-list-show-packages)
  (guix-generation-list-show-added-packages)
  (guix-generation-list-show-removed-packages)
  (guix-generation-list-diff, guix-generation-list-diff-manifests)
  (guix-generation-list-ediff, guix-generation-list-ediff-manifests)
  (guix-generation-list-diff-packages)
  (guix-generation-list-ediff-packages)
  (guix-generation-list-mark-delete, guix-generation-list-execute): Move
  to "guix-ui-generation.el".
* emacs/guix.el: Remove.
  (guix, guix-faces, guix-edit): Move to "guix-base.el".
  (guix-list-single-package, guix-search-params, guix-search-history)
  (guix-get-show-packages, guix-search-by-name, guix-search-by-regexp)
  (guix-installed-packages, guix-obsolete-packages)
  (guix-all-available-packages, guix-newest-available-packages): Move
  to "guix-ui-package.el".
  (guix-get-show-generations, guix-generations, guix-last-generations)
  (guix-generations-by-time): Move to "guix-ui-generation.el".
* emacs.am (ELFILES): Remove "guix.el".  Add "guix-ui-package.el" and
  "guix-ui-generation.el".
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.