Doc Fix? Setting STAN_THREADS=false (or FALSE, 0, etc) results in STAN_THREADS=true #1293

Summary:

As concisely stated in this comment: #1179 (comment)

Most of our makefile logic just checks if STAN_THREADS is defined, not that it is set to a "truthy" value.

However, the documentation does not explicitly state this, leading users (including apparently cmdstanr developers) to incorrectly infer that falsy values will turn off multi-threading.

Description:

Not sure if the behavior should change or the doc should change here, but certainly if the behavior remains as-is, it would be good to make the doc more explicit. This issue likely impacts other flags as well.

It would also be helpful for make help-dev or a similar command to resolve the value to how it will be interpreted at compile time.

Reproducible Steps:

~/.cmdstan/cmdstan-2.35.0$ STAN_THREADS=false make examples/bernoulli/bernoulli
~/.cmdstan/cmdstan-2.35.0$ examples/bernoulli/bernoulli info|grep STAN_THREADS

Current Output:

STAN_THREADS=true

Expected Output:

STAN_THREADS=false

Current Version:

v2.35.0