Allow make -B / make DEBUG=1 -B to run without errors.

GNUmakefile

@@ -173,13 +173,13 @@ OBJECTS := ${OBJDIR}ODAuto${OBJFILE}\
 #         ${OBJDIR}odsys${OBJFILE}\	this file is missing
 
 ${OBJDIR}:
-	mkdir ${OBJDIR}
+	mkdir -p ${OBJDIR}
 
 ${LIBDIR}:
-	mkdir ${LIBDIR}
+	mkdir -p ${LIBDIR}
 
 ${EXEDIR}:
-	mkdir ${EXEDIR}
+	mkdir -p ${EXEDIR}
 
 $(OBJDIR)%$(OBJFILE) : %.c
 	$(CC) $(CFLAGS) -o $@ -c $<