message: if STANC3_TEST_BIN_URL is set, use that for download
diff: @@ -12,6 +12,7 @@ OS ?= missing-submodules
  CMDSTAN_SUBMODULES = 1
  STANC_DL_RETRY = 5
  STANC_DL_DELAY = 10
  +STANC3_TEST_BIN_URL ?=
  ifeq ($(OS),Windows_NT)
  OS_TAG := windows
  @@ -33,7 +34,17 @@ ifneq ($(STANC3),)
  @mkdir -p $(dir $@)
  cd $(STANC3) && echo "--- Rebuilding stanc ---\n" && dune build @install
  cp $(STANC3)/_build/default/src/stanc/stanc.exe $@
  -
  +else ifneq ($(STANC3_TEST_BIN_URL),)
  +ifeq ($(OS_TAG),windows)
  + bin/stanc$(EXE) :
  + @mkdir -p $(dir $@)
  + $(shell echo "curl -L $(STANC3_TEST_BIN_URL)/bin/$(OS_TAG)-stanc -o bin/stanc$(EXE) --retry $(STANC_DL_RETRY) --retry-delay $(STANC_DL_DELAY)")
  +else
  + bin/stanc$(EXE) :
  + @mkdir -p $(dir $@)
  + curl -L $(STANC3_TEST_BIN_URL)/bin/$(OS_TAG)-stanc -o bin/stanc$(EXE) --retry $(STANC_DL_RETRY) --retry-delay $(STANC_DL_DELAY)
  + chmod +x bin/stanc$(EXE)
  +endif
  else ifneq (,$(wildcard bin/$(OS_TAG)-stanc))
  bin/stanc$(EXE) :
  cp bin/$(OS_TAG)-stanc bin/stanc$(EXE)
  