more precise dtx build rule

Makefile

@@ -29,7 +29,8 @@ all: pdf bib
 	$(PDFLATEX) $(TEX_FLAGS) $(QUIET) $(NAME).tex
 	$(PDFLATEX) -synctex=1 $(TEX_FLAGS) $(QUIET) $(NAME).tex
 
-src: HsH-Classes.ins HsH-Classes.dtx
+src: src/HsH-article.cls src/HsH-report.cls src/HsH-standalone.cls
+src/HsH-article.cls src/HsH-report.cls src/HsH-standalone.cls &: HsH-Classes.ins HsH-Classes.dtx
 	$(LATEX) -output-directory=src HsH-Classes.ins
 
 pdf: $(SUB_FILES) $(NAME).pdf