Merge branch 'master' into core-updates

Conflicts:
	Makefile.am
	guix/scripts/gc.scm
	guix/scripts/package.scm
	guix/ui.scm
	tests/guix-package.sh

Add (guix build-system perl).

* guix/build-system/perl.scm, guix/build/perl-build-system.scm: New files.
* Makefile.am (MODULES): Add them.
* guix/build-system/gnu.scm (standard-inputs): Make public.