Add -p to mkdir call

Author: WardBrian

make/program

@@ -24,7 +24,7 @@ $(patsub %.d,%.hpp.gch,$(PRECOMPILED_MODEL_HEADER)) : DEPTARGETS = -MT $(patsubs
 $(PRECOMPILED_MODEL_HEADER) : $(STAN)src/stan/model/model_header.hpp
 	@echo ''
 	@echo '--- Compiling pre-compiled header. This might take a few seconds. ---'
-	@mkdir $(dir $@)
+	@mkdir -p $(dir $@)
 	$(COMPILE.cpp) $< $(OUTPUT_OPTION)
 
 ifeq ($(CXX_TYPE),clang)