scripts: Set the compiler warning options to "-Wall -Wextra -Wundef"

Considering that "-Wextra" is a more descriptive alternative to "-W",
and that "-Wundef" is a highly useful warning option that has been
available in ancient versions of gcc (version 2.x), we replace all
occurrences of "-W -Wall" with "-Wall -Wextra -Wundef".

Also clean up the makefiles.

scripts/makefile.dj2

@@ -1,4 +1,5 @@
 # DJGPP (DOS gcc) makefile for libpng
+# Copyright (C) 2020-2022 Cosmin Truta
 # Copyright (C) 2002, 2006, 2009-2014 Glenn Randers-Pehrson
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
 #
@@ -17,8 +18,6 @@ CPPFLAGS=-I../zlib -DPNG_NO_SNPRINTF
 CFLAGS=-O
 LDFLAGS=-L. -L../zlib/ -lpng -lz -lm
 
-RANLIB=ranlib
-
 CP=cp
 RM_F=rm -f
 
@@ -39,8 +38,8 @@ pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
 	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 libpng.a: $(OBJS)
-	ar rc $@  $(OBJS)
-	$(RANLIB) $@
+	ar rc $@ $(OBJS)
+	ranlib $@
 
 pngtest: pngtest.o libpng.a
 	$(CC) -o pngtest $(CFLAGS) pngtest.o $(LDFLAGS)