make doesn't manage dependencies of Stan library correctly when STAN_THREADS=true

[bbbales2]

Summary:

With STAN_THREADS=true in make/local, if you:

1. Build a model
2. Modify the stan submodule source
3. Rebuild the model

Nothing happens on step 3.

If threading is off, this works fine. (Edit: and by fine, I mean step 3 rebuilds the model because a dependency in the Stan submodule changed)

I suspect this has something to do with when STAN_THREADS=true src/cmdstan/main.cpp gets built into src/cmdstan/main_threads.o instead of src/cmdstan/main.o

I'm not sure what the fix is, but this is a minor bug with the makefiles. It's easy enough to work around in my case (I'll just do my development here with non-threaded builds)

Current Version:

v2.26.1