[Rd] Detect UCRT-built R from within R sessions (and in configure.win)

[Hiroaki Yutani]

Hi,

Are there any proper ways to know whether the session is running on
the R that is built with the UCRT toolchain or not? Checking if the
encoding is UTF-8 might do the trick, but I'm not sure if it's always
reliable.

Also, I'd like to know if there's any mechanism to detect the UCRT in
configure.win. I know there are Makevars.ucrt and Makefile.ucrt, but
one might want to do some feature test that is specific to the UCRT
toolchain.

Best,
Hiroaki Yutani