Only add flag for UCRT

Author: andrjohns

make/tests

@@ -101,12 +101,15 @@ HEADER_TESTS := $(addsuffix -test,$(call findfiles,stan,*.hpp))
 
 ifeq ($(OS),Windows_NT)
   DEV_NULL = nul
+  ifeq ($(IS_UCRT),true)
+    UCRT_NULL_FLAG = -S
+  endif
 else
   DEV_NULL = /dev/null
 endif
 
 %.hpp-test : %.hpp test/dummy.cpp
-	$(COMPILE.cpp) $(CXXFLAGS) -O0 -include $^ -S -o $(DEV_NULL) -Wunused-local-typedefs
+	$(COMPILE.cpp) $(CXXFLAGS) -O0 -include $^ $(UCRT_NULL_FLAG) -o $(DEV_NULL) -Wunused-local-typedefs
 
 test/dummy.cpp:
 	@mkdir -p test