* guix/build-system/r.scm (bioconductor-uri): New procedure.
* guix/build-system/r.scm (cran-uri): New procedure.
* guix/build-system/r.scm: New file. * guix/build/r-build-system: New file. * Makefile.am (MODULES): Add new files. * doc/guix.texi (Build Systems): Document r-build-system.