The python-build-system uses phases the build and install, but not
configure. So the old comment was plain wrong since Sept. 2013, when the build
phase has been added.
This phase is inherited by other build systems, which ensures
'SOURCE_DATE_EPOCH' is always set in practice.
* guix/build/python-build-system.scm (set-SOURCE-DATE-EPOCH): Remove.
(%standard-phases): Don't add it.
* guix/build/gnu-build-system.scm (set-SOURCE-DATE-EPOCH): New
procedure.
(%standard-phases): Add it.
(gnu-build): Remove 'setenv' call for "SOURCE_DATE_EPOCH".
* 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/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.
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.