Fix for parallel build

src/Makefile

@@ -18,13 +18,22 @@ include $(DEPENDS)
 %.d: %.cpp
 	 $(CC) $(CFLAGS) -MM -MT $(subst .cpp,.o,$<) $< | sed 's%\(^.*\):%\1 $@ :%g' >$@
 
+$(DEPENDS): parser/grammatical.h parser/lexical.h
+
+
 parser/grammatical.d: parser/lexical.h
 
-parser/lexical.h parser/lexical.cpp: parser/lexical.l
+parser/lexical.cpp: parser/lexical.l
 	 cd parser && flex lexical.l
+	 [ -f parser/lexical.h ] && touch parser/lexical.h
 
-parser/grammatical.h parser/grammatical.cpp: parser/grammatical.y
+parser/grammatical.cpp: parser/grammatical.y
 	 cd parser && bison grammatical.y
+	 [ -f parser/grammatical.h ] && touch parser/grammatical.h
+
+parser/grammatical.h: parser/grammatical.y parser/grammatical.cpp
+parser/lexical.h: parser/lexical.l parser/lexical.cpp
+
 
 clean:
 	 rm -f *.o *.d parser/*.{o,d} parser/{lexical,grammatical}.{cpp,h}