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.so9

@@ -1,7 +1,7 @@
 # makefile for libpng on Solaris 9 (beta) with Forte cc
 # Updated by Chad Schrock for Solaris 9
 # Contributed by William L. Sebok, based on makefile.linux
-# Copyright (C) 2020 Cosmin Truta
+# Copyright (C) 2020-2022 Cosmin Truta
 # Copyright (C) 2002, 2006, 2008, 2010-2014 Glenn Randers-Pehrson
 # Copyright (C) 1998-2001 Greg Roelofs
 # Copyright (C) 1996-1997 Andreas Dilger
@@ -24,9 +24,9 @@ OLDSO=libpng.so
 # gcc 2.95 doesn't work.
 CC=cc
 AR_RC=ar rc
+RANLIB=echo
 MKDIR_P=mkdir -p
 LN_SF=ln -f -s
-RANLIB=echo
 CP=cp
 RM_F=/bin/rm -f
 
@@ -45,11 +45,7 @@ exec_prefix=$(prefix)
 ZLIBLIB=/usr/lib
 ZLIBINC=/usr/include
 
-#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 -O3 $(WARNMORE) -g
 CFLAGS=-O3
 LDFLAGS=-L. -R. -L$(ZLIBLIB) -R$(ZLIBLIB) -lpng16 -lz -lm