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.sgi

@@ -1,5 +1,5 @@
 # makefile for libpng.a and libpng16.so, SGI IRIX with 'cc'
-# Copyright (C) 2020 Cosmin Truta
+# Copyright (C) 2020-2022 Cosmin Truta
 # Copyright (C) 2001-2002, 2006, 2007, 2010-2014 Glenn Randers-Pehrson
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
 #
@@ -18,11 +18,11 @@ LIBSOREL=$(LIBSOMAJ).$(RELEASE)
 OLDSO=libpng.so
 
 # Utilities:
-AR_RC=ar rc
 CC=cc
+AR_RC=ar rc
+RANLIB=echo
 MKDIR_P=mkdir -p
 LN_SF=ln -sf
-RANLIB=echo
 CP=cp
 RM_F=/bin/rm -f
 
@@ -48,9 +48,9 @@ WARNMORE=-fullwarn
 # Note: -KPIC is the default anyhow
 CPPFLAGS=-I$(ZLIBINC) # -DPNG_DEBUG=5
 #CFLAGS= $(ABI) -O $(WARNMORE) -KPIC # -g
-CFLAGS=$(ABI) -O $(WARNMORE)
-LDFLAGS_A=$(ABI) -L. -L$(ZLIBLIB) -lpng16 -lz -lm
-LDFLAGS=$(ABI) -L. -L$(ZLIBLIB) -lpng -lz -lm
+CFLAGS=$(ABI) -O $(WARNMORE) # -g
+LDFLAGS_A=$(ABI) -L. -L$(ZLIBLIB) -lpng16 -lz -lm # -g
+LDFLAGS=$(ABI) -L. -L$(ZLIBLIB) -lpng -lz -lm # -g
 LDSHARED=cc $(ABI) -shared -soname $(LIBSOMAJ) \
 	-set_version sgi$(PNGMAJ).0
 # See "man dso" for info about shared objects