gnu: Add ruby-sdoc.

* gnu/packages/ruby.scm (ruby-sdoc): New variable.

gnu: Add ruby-pry-editline.

* gnu/packages/ruby.scm (ruby-pry-editline): New variable.

gnu: Add ruby-ci-reporter.

* gnu/packages/ruby.scm (ruby-ci-reporter): New variable.

gnu: Add ruby-atoulme-saikuro.

* gnu/packages/ruby.scm (ruby-atoulme-saikuro): New variable.

gnu: Add ruby-simplecov.

* gnu/packages/ruby.scm (ruby-simplecov): New variable.

gnu: Add sparsehash.

* gnu/packages/crypto.scm (sparsehash): New variable.

gnu: Add cereal.

* gnu/packages/serialization.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.

gnu: xorriso: Update to 1.4.2.

* gnu/packages/cdrom.scm (xorriso): Update to 1.4.2.

gnu: python: Move tkinter to "tk" output.

* gnu/packages/python.scm (python-2)[outputs]: New field.
[arguments]: Add 'move-tk-inter' phase.
(python2-minimal, python-minimal, wrap-python3)[outputs]: New field.

gnu: python: Clarify and shorten the list of built-time modules.

* gnu/packages/python.scm (python-2)[arguments]: Make the #:modules list
  shorter.

gnu: python: Do not record configure flags.

This reduces the number of run-time dependencies of python-2 from 25 to
15, corresponding only to ~4 MiB.

* gnu/packages/python.scm (python-2)[arguments]: Add
  'do-not-record-configure-flags' phase.

gnu: python: Factorize configure flags with minimal variants.

* gnu/packages/python.scm (python2-minimal, python-minimal)[arguments]:
  Reuse the inherited configure flags, and just add
  --without-system-ffi.

gnu: python: Make 'sys.version' deterministic.

* gnu/packages/patches/python-2-deterministic-build-info.patch,
  gnu/packages/patches/python-3-deterministic-build-info.patch: New
  files.
* gnu/packages/python.scm (python-2, python): Use them.
* gnu-system.am (dist_patch_DATA): Add them.

gnu: python: Set SOURCE_DATE_EPOCH to 1 instead of 0.

* gnu/packages/python.scm (python-2)[arguments]: Set SOURCE_DATE_EPOCH
  to 1 to match what the daemon does.
* guix/build/python-build-system.scm (set-SOURCE-DATE-EPOCH): Likewise.

gnu: python-2: Honor 'SOURCE_DATE_EPOCH'.

* gnu/packages/patches/python-2.7-source-date-epoch.patch: New file.
* gnu/packages/python.scm (python-2)[source]: Use it.
  [arguments]: Set SOURCE_DATE_EPOCH in 'patch-lib-shells' phase.
* guix/build/python-build-system.scm (set-SOURCE-DATE-EPOCH): New
  procedure.
  (%standard-phases): Add it.
* gnu-system.am (dist_patch_DATA): Add patch.

gnu: tcl: Produce 'tclIndex' files deterministically.

* gnu/packages/patches/tcl-mkindex-deterministic.patch: New patch.
* gnu/packages/tcl.scm (tcl)[source]: Use it.
* gnu-system.am (dist_patch_DATA): Add it.

tk: Hardcode path to TK_LIBRARY.

Fixes <http://bugs.gnu.org/20889>.

* gnu/packages/patches/tk-find-library.patch: New patch.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/tcl.scm (tk)[source]: Add patch.

refresh: Check updater availability at run time.

This is a followup to b68d2db, which added a check for updaters at
macro-expansion time.  The problem is that, when running 'guix pull',
Guile-JSON is found, so the PyPi updater (say) is added to %UPDATERS,
but then at run time Guile-JSON might be missing.

Reported by orbea on #guix.

* guix/scripts/refresh.scm (maybe-updater): Rewrite as 'syntax-rules'.
Produce code that checks conditions at run time.
(list-updaters): Update docstring.

gnu: Add perl-dbd-mysql.

* gnu/packages/databases.scm (perl-dbd-mysql): New variable.

gnu: Add Rakarrack.

* gnu/packages/audio.scm (rakarrack): New variable.