* guix/build/guile-build-system.scm (build): Use invoke-each, instead of for-each, to use multiple cores if available. (invoke-each, report-build-process): New procedures.
* guix/build-system/guile.scm, guix/build/guile-build-system.scm: New files. * Makefile.am (MODULES): Add them. * doc/guix.texi (Build Systems): Document 'guile-build-system'.