CRAN URL checks

The CRAN submission checks run by R CMD check --as-cran check the availability of URLs in files including DESCRIPTION, CITATION, NEWS.Rd, the .Rd help pages and HTML files in inst/doc, plus NEWS.md and README.md in R-devel (3.3.0-to-be). These notes explain the checks.

They are only done if the build of R in use supports a suitably recent version of libcurl. Most should, but OS X ‘Snow Leopard’ and Linux distributions from before 2013 will be too old to do so.