Warning - 'config' variable 'CPP' is deprecated

Summary:

After encountering #774 and installing this version of rstan I get a 'config' variable 'CPP' is deprecated warning message / error. I think this is happening here. It looks like this was depreciated in R 3.6.2: https://cran.r-project.org/bin/windows/base/old/3.6.2/NEWS.R-3.6.2.html

Everything Stan related still works as it should, but it makes some of my package's tests fail.

Reproducible Steps:

library(rstan)

res <- stan_model(
  model_code = "data {} parameters {real x;} model {x ~ std_normal();}"
)

gives :

 'config' variable 'CPP' is deprecated
clang -mmacosx-version-min=10.13 -E
Warning message:
In system(paste(CPP, ARGS), ignore.stdout = TRUE, ignore.stderr = TRUE) :
  error in running command

RStan Version:

‘2.21.3’, 27a744a

R Version:

"R version 4.0.3 (2020-10-10)"

Operating System:

OS X 10.15.7

clang info

$ clang --version
Apple clang version 12.0.0 (clang-1200.0.32.27)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Start by reproducing the warning with the stan_model example on R 4.0.3 and macOS, then inspect the config handling at rstan/rstan/R/stanc.R lines 123-124. Verify that the model still compiles and the deprecated CPP warning and command error no longer appear.