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.64sunu

@@ -1,6 +1,6 @@
 # makefile for libpng on Solaris 2.x with cc
 # Contributed by William L. Sebok, based on makefile.linux
-# Copyright (C) 2020 Cosmin Truta
+# Copyright (C) 2020-2022 Cosmin Truta
 # Copyright (C) 2002, 2006, 2010-2014 Glenn Randers-Pehrson
 # Copyright (C) 1998 Greg Roelofs
 # Copyright (C) 1996, 1997 Andreas Dilger
@@ -22,9 +22,9 @@ OLDSO=libpng.so
 # Utilities:
 CC=cc
 AR_RC=ar rc
+RANLIB=echo
 MKDIR_P=mkdir -p
 LN_SF=ln -f -s
-RANLIB=echo
 RM_F=/bin/rm -f
 
 SUN_CC_FLAGS=-fast -xtarget=ultra -xarch=v9
@@ -45,7 +45,7 @@ ZLIBINC=/usr/include
 
 WARNMORE=-Wwrite-strings -Wpointer-arith -Wshadow \
 	-Wmissing-declarations -Wtraditional -Wcast-align \
-	-Wstrict-prototypes -Wmissing-prototypes #-Wconversion
+	-Wstrict-prototypes -Wmissing-prototypes # -Wconversion
 CPPFLAGS=-I$(ZLIBINC) # -DPNG_DEBUG=5
 CFLAGS= $(SUN_CC_FLAGS) # $(WARNMORE) -g
 LDFLAGS=-L. -R. $(SUN_LD_FLAGS) -L$(ZLIBLIB) -R$(ZLIBLIB) -lpng16 -lz -lm