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.solaris

@@ -1,5 +1,5 @@
 # makefile for libpng on Solaris 2.x with gcc
-# Copyright (C) 2020 Cosmin Truta
+# Copyright (C) 2020-2022 Cosmin Truta
 # Copyright (C) 2004, 2006-2008, 2010-2014 Glenn Randers-Pehrson
 # Contributed by William L. Sebok, based on makefile.linux
 # Copyright (C) 1998 Greg Roelofs
@@ -20,11 +20,11 @@ LIBSOREL=$(LIBSOMAJ).$(RELEASE)
 OLDSO=libpng.so
 
 # Utilities:
-AR_RC=ar rc
 CC=gcc
+AR_RC=ar rc
+RANLIB=echo
 MKDIR_P=mkdir -p
 LN_SF=ln -f -s
-RANLIB=echo
 CP=cp
 RM_F=/bin/rm -f
 
@@ -43,10 +43,10 @@ ZLIBINC=/usr/local/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=-W -Wall -O # $(WARNMORE) -g
-LDFLAGS=-L. -R. -L$(ZLIBLIB) -R$(ZLIBLIB) -lpng16 -lz -lm
+CFLAGS=-O -Wall -Wextra -Wundef # $(WARNMORE) -g
+LDFLAGS=-L. -R. -L$(ZLIBLIB) -R$(ZLIBLIB) -lpng16 -lz -lm # -g
 
 INCPATH=$(prefix)/include
 LIBPATH=$(exec_prefix)/lib