build: Always check for gzip/bzip2/xz.

This allows (guix config) to contain valid values of %GZIP et al. even
when configured with --disable-daemon.

* config-daemon.ac: Move 'AC_PATH_PROG' invocations for
gzip & co. to...
* configure.ac: ... here.

gnu: youtube-dl: Update to 2015.12.09.

* gnu/packages/video.scm (youtube-dl): Update to 2015.12.09.
[native-inputs]: Move python-setuptools here.

gnu: vte: Update to 0.42.1.

* gnu/packages/gnome.scm (vte): Update to 0.42.1.

gnu: libsecret: Update to 0.18.3.

* gnu/packages/gnome.scm (libsecret): Update to 0.18.3.

gnu: libgnome-keyring: Update to 3.12.0.

* gnu/packages/gnome.scm (libgnome-keyring): Update to 3.12.0.

emacs: Fix '--rounds' option in command popups.

* emacs/guix-command.el (guix-command-improve-common-build-argument):
  Use "R" key for '--rounds' option to avoid conflicts with "r" key in
  'build' popup (--root) and 'package' popup (--remove).

emacs: Add 'guix-build-log-find-file'.

* emacs/guix-build-log.el (guix-build-log-find-file): New procedure.
* emacs/guix-command.el (guix-run-view-build-log): Use it.

emacs: list: Fontify time stamps.

* emacs/guix-list.el (guix-list-time): New face.
  (guix-list-get-time): Use it.

emacs: info: Add 'guix-info-heading' face.

* emacs/guix-info.el (guix-info-heading): New face.
  (guix-package-info-heading): Inherit it.

Add "Contributor Covenant".

* CODE-OF-CONDUCT: New file, adapted from
  <http://contributor-covenant.org/version/1/3/0/code_of_conduct.txt>.
* doc/contributing.texi (Contributing): Mention it.
* Makefile.am (EXTRA_DIST): Add it.

daemon: Use deterministic $TMPDIR in chroot.

Rather than using $<host-TMPDIR>/nix-build-<drvname>-<number>, the
temporary directory is now always /tmp/nix-build-<drvname>-0. This
improves bitwise-exact reproducibility for builds that store $TMPDIR
in their build output. (Of course, those should still be fixed...)

* nix/libstore/build.cc (DerivationGoal)[tmpDirInSandbox]: New field.
(DerivationGoal::startBuilder): Initialize 'useChroot' earlier.  Compute
'tmpDirInSandbox', and use it when populating 'dirsInChroot'.
* doc/guix.texi (Build Environment Setup): Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

guix build: Add '--check'.

* guix/derivations.scm (build-derivations): Add optional 'mode'
parameter.
* guix/scripts/build.scm (%default-options): Add 'build-mode'.
(show-help, %options): Add '--check'.
(guix-build): Honor 'build-mode' key of OPTS.  Pass it to
'show-what-to-build' and 'build-derivations'.
* doc/guix.texi (Invoking guix build): Document it.
(Substitutes): Mention it.

derivations: Determine what's built in 'check' mode.

* guix/derivations.scm (substitution-oracle): Add #:mode parameter and
honor it.
(derivation-prerequisites-to-build): Likewise.
[derivation-built?]: Take it into account.
* guix/ui.scm (show-what-to-build): Add #:mode parameter.  Pass it to
'substitute-oracle' and 'derivations-prerequisites-to-build'.
* tests/derivations.scm ("derivation-prerequisites-to-build in 'check'
mode"): New test.

gnu: gmime: Update to 2.6.20.

* gnu/packages/mail.scm (gmime): Update to 2.6.20.
[arguments]: Use modify-phases syntax.

gnu: guile-sly: Use HTTPS URL.

* gnu/packages/guile.scm (guile-sly)[source]: Use 'https'.  Failing to
do that, we get redirected to 'https', which fails because GnuTLS is
missing.

guix build: Add '--rounds'.

* guix/scripts/build.scm (show-build-options-help)
(%standard-build-options): Add --rounds.
(set-build-options-from-command-line): Honor it.
* doc/guix.texi (Invoking guix build): Document it.
* doc/contributing.texi (Submitting Patches): Mention it.

store: Allow clients to request multiple builds.

* guix/store.scm (set-build-options): Add #:rounds parameter and honor it.
* tests/store.scm ("build multiple times"): New test.

daemon: Allow builds to be repeated.

This makes it easy to detect non-deterministic builds.

* nix/libstore/build.cc (DerivationGoal): Remove 'InodesSeen'; add
'curRound', 'nrRound', and 'prevInfos'.
(DerivationGoal::inputsRealised): Initialize 'nrRound'.
(NotDeterministic): New error type.
(DerivationGoal::buildDone): Check whether we need to repeat.
(DerivationGoal::startBuilder): Adjust message.
(DerivationGoal::registerOutputs): Check whether we get the same result.
* nix/libstore/globals.cc (Settings::get(const string & name, int def)):
New method.
* nix/libstore/globals.hh (Settings): Add it.
* nix/libstore/store-api.hh (ValidPathInfo): Add operator ==.
* nix/nix-daemon/nix-daemon.cc (performOp): Allow "build-repeat" for
"untrusted" users.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

gnu: python-pillow: Update to 3.0.0.

* gnu/packages/python.scm (python-pillow, python2-pillow): Update to 3.0.0.

gnu: numactl: Update to 2.0.10.

* gnu/packages/linux.scm (numactl)[source]: Update to 2.0.10.
  [arguments]: Adapt phases to use the autotools machinery.
  [native-inputs]: Add autotools packages.