Log Message:
(lang/R-cpp11) Updated 0.5.0 to 0.5.1

# cpp11 0.5.1

* cpp11 now requires R >=4.0.0, in line with the
  [tidyverse version \ 
policy](https://www.tidyverse.org/blog/2019/04/r-version-support/) (#411).

* Because cpp11 now requires R >=4.0.0, a number of previously optional tools
  are now always available, allowing us to remove some dead code. In
  particular:

  * `R_UnwindProtect()` is always available, so the defines `HAS_UNWIND_PROTECT`
    and `CPP11_UNWIND` are no longer useful.

  * ALTREP is always available, so the file `cpp11/altrep.hpp` and the define
    `HAS_ALTREP` are no longer useful.

  We would like to remove the dead code regarding these tools in the future, so
  we ask that you please remove usage of them from your own packages (#411).

* `R_NO_REMAP` and `STRICT_R_HEADERS` are now conditionally defined only if they
  have not already been defined elsewhere. This is motivated by the fact that
  `R_NO_REMAP` is becoming the default for C++ code in R 4.5.0 (#410).

* Fixed a small protection issue flagged by rchk (#408).