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.gcc

@@ -1,6 +1,6 @@
 # makefile for libpng using gcc (generic, static library)
+# Copyright (C) 2000, 2014, 2019-2022 Cosmin Truta
 # Copyright (C) 2008, 2014 Glenn Randers-Pehrson
-# Copyright (C) 2000, 2014, 2019 Cosmin Truta
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
 #
 # This code is released under the libpng license.
@@ -23,8 +23,8 @@ WARNMORE = -Wwrite-strings -Wpointer-arith -Wshadow \
 	-Wmissing-declarations -Wtraditional -Wcast-align \
 	-Wstrict-prototypes -Wmissing-prototypes # -Wconversion
 CPPFLAGS = -I$(ZLIBINC) # -DPNG_DEBUG=5
-CFLAGS = -W -Wall -O2 # $(WARNMORE) -g
-LDFLAGS = -L$(ZLIBLIB)
+CFLAGS = -O2 -Wall -Wextra -Wundef # $(WARNMORE) -g
+LDFLAGS = -L$(ZLIBLIB) # -g
 LIBS = -lz -lm
 
 # File extensions