* guix/build-system/copy.scm (copy-build): Set install-plan default value to copy everything from source to the output.
* guix/build-system/copy.scm: New file. * guix/build/copy-build-system.scm: New file. * Makefile.am (MODULES): Add them. * doc/guix.texi (Build Systems): Document 'copy-build-system'.