add unit test for recursive make support

tests/testthat/test-utils.R

@@ -248,3 +248,16 @@ test_that("as_mcmc.list() works", {
     "Currently only CmdStanMCMC objects can be converted to mcmc.list"
   )
 })
+
+test_that("get_cmdstan_flags() can be used recursively in `make`", {
+  tmp <- withr::local_tempfile()
+  writeLines(
+    "test:\n\t@Rscript --vanilla -e 'cat(cmdstanr:::get_cmdstan_flags(\"STANCFLAGS\"))'",
+    tmp
+  )
+  nonrecursive_flags <- get_cmdstan_flags("STANCFLAGS")
+  recursive_flags <- readLines(textConnection(wsl_compatible_run(
+    command = "make", args = c("-f", tmp)
+  )$stdout))
+  expect_equal(nonrecursive_flags, recursive_flags)
+})