derivations: Use more keyword parameters for 'build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Turn 'system' and
  'inputs' into keyword parameters.
  Adjust callers accordingly.
* gnu/system/linux.scm, gnu/system/vm.scm, guix/build-system/cmake.scm,
  guix/build-system/gnu.scm, guix/build-system/perl.scm,
  guix/build-system/python.scm, guix/build-system/trivial.scm,
  guix/download.scm, guix/packages.scm, guix/profiles.scm,
  guix/scripts/pull.scm, tests/derivations.scm, tests/guix-build.sh,
  tests/monads.scm, tests/store.scm, tests/union.scm: Adjust users of
  'build-expression->derivation' and 'derivation-expression'
  accordingly.
* doc/guix.texi (Derivations): Adjust 'build-expression->derivation'
  documentation accordingly.
  (The Store Monad): Likewise for 'derivation-expression'.

Merge branch 'core-updates'

Conflicts:
	gnu/packages/gnunet.scm

Merge branch 'master' into core-updates

Conflicts:
	guix/packages.scm

build-system/python: Add #:test-target parameter.

* guix/build-system/python.scm (python-build): Add #:test-target
  parameter.  Reported by Eric Bavier <ericbavier@gmail.com>.

build-system/{gnu,cmake}: Remove #:patches and #:patch-flags parameters.

* guix/build/gnu-build-system.scm (patch): Remove.
  (%standard-phases): Remove 'patch'.
* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Remove
  #:patches and #:patch-flags parameters.
* guix/build-system/cmake.scm (cmake-build): Likewise.
* guix/build-system/python.scm (package-with-explicit-python): Update
  comment.

Merge branch 'master' into core-updates

derivations: 'derivation' and related procedures return a single value.

* guix/derivations.scm (derivation->output-path,
  derivation->output-paths): New procedures.
  (derivation-path->output-path): Use 'derivation->output-path'.
  (derivation-path->output-paths): Use 'derivation->output-paths'.
  (derivation): Accept 'derivation?' objects as inputs.  Return a single
  value.
  (build-derivations): New procedure.
  (compiled-modules): Use 'derivation->output-paths'.
  (build-expression->derivation)[source-path]: Add case for when the
  input matches 'derivation?'.
  [prologue]: Accept 'derivation?' objects in INPUTS.
  [mod-dir, go-dir]: Use 'derivation->output-path'.
* guix/download.scm (url-fetch): Adjust to the single-value return.
* guix/packages.scm (package-output): Use 'derivation->output-path'.
* guix/scripts/build.scm (guix-build): When the argument is
  'derivation-path?', pass it through 'read-derivation'.
  Use 'derivation-file-name' to print out the .drv file names, and to
  register them.  Use 'derivation->output-path' instead of
  'derivation-path->output-path'.
* guix/scripts/package.scm (roll-back): Adjust to the single-value
  return.
  (guix-package): Use 'derivation->output-path'.
* guix/ui.scm (show-what-to-build): Adjust to deal with 'derivation?'
  objects instead of .drv file names.
* gnu/system/grub.scm (grub-configuration-file): Use
  'derivation->output-path' instead of 'derivation-path->output-path'.
* gnu/system/vm.scm (qemu-image, system-qemu-image): Likewise.
* tests/builders.scm, tests/derivations.scm, tests/packages.scm,
  tests/store.scm, tests/union.scm: Adjust to the new calling
  convention.
* doc/guix.texi (Defining Packages, The Store, Derivations): Adjust
  accordingly.

Merge branch 'python'

guix: python: Do not import %standard-phases from gnu-build-system.

* guix/build-system/python.scm (python-build): Drop module gnu-build-system.
    Thanks to Ludovic Courtès <ludo@gnu.org>.

guix: python: Add parameter #:phases to build system.

* guix/build-system/python.scm (python-build): Use parameter #:phases.

guix: python: Add package-with-python2, a procedure rewriting a package
  to compile with Python 2 instead of the default Python 3.

* guix/build-system/python.scm (default-python2, package-with-explicit-python,
    package-with-python2): New procedures.
* guix/build-system/python.scm (python2-pytz, python2-babel): Use
    package-with-python2.

guix: python: Switch to python-wrapper as the default version for the python
  build system (switches to Python 3) and compute python-version instead of
  passing it as a parameter.

* guix/build-system/python.scm (default-python): Switch to python-wrapper.
* guix/build-system/python.scm (python-build): Drop parameter #:python-version.
* guix/build/python-build-system.scm (wrap): Compute python version from input.

build-system/python: Avoid circular dependency.

This fixes a circular dependency introduced in commit acc26ff ("gnu: Add
pytz.").

* guix/build-system/python.scm (default-python): New procedure.
  (python-build): Use it.

Add 'python-build-system'.

* guix/build-system/python.scm, guix/build/python-build-system.scm: New files.
* Makefile.am (MODULES): Add them.