Clarify the necessary "move" w.r.t. `SystemRequirements: C++11`

[jennybc]

Currently cpp11's README and, therefore, home page says:

... add the following to your DESCRIPTION file:

LinkingTo: cpp11
SystemRequirements: C++11

But as we all know too well, CRAN is asking for packages to drop SystemRequirements: C++11 in general.

Should this advice be changed in cpp11?

My immediate motivation for opening this issue is to do my due diligence before I modify usethis::use_cpp11() to stop adding SystemRequirements: C++11 to DESCRIPTION (r-lib/usethis#1824).

[stephematician]

As someone who just submitted their first package ... yeah, that was a weird hurdle. Would make life easier if the advice was updated.

Re some of the advice in that tidyverse blog: I'm not sure I agree that CXX_STD=CXX11 should be removed from Makevars seeing as some older compilers might not default to the standard - so it can't hurt to give them a chance. I'd prefer to leave it in, and CRAN doesn't seem to mind.

[jennybc]

Since I opened this issue, I made the executive decision for usethis::use_cpp11() to stop putting C++11 in SystemRequirements. That is currently only the dev version of usethis, but there will be a release soon-ish.

r-lib/usethis@4a83ded