WISH: Report on existence/usage of ~/.R/check.Renviron

[HenrikBengtsson]

For troubleshooting purposes, having revdep_check() report:

if (file_test("-f", p <- Sys.getenv("R_CHECK_ENVIRON", "~/.R/check.Renviron")))
  cat("R CMD check will use env vars from", sQuote(p), "\n")

would be useful, e.g. Issue #136.

PS. It looks like you want to use stdout, so I used cat() above.