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.linux

@@ -1,5 +1,5 @@
 # makefile for libpng.a and libpng16.so on Linux ELF with gcc
-# Copyright (C) 2020 Cosmin Truta
+# Copyright (C) 2020-2022 Cosmin Truta
 # Copyright (C) 1998, 1999, 2002, 2006, 2008, 2010-2014 Greg Roelofs and
 # Glenn Randers-Pehrson
 # Copyright (C) 1996, 1997 Andreas Dilger
@@ -20,13 +20,13 @@ LIBSOREL=$(LIBSOMAJ).$(RELEASE)
 OLDSO=libpng.so
 
 # Utilities:
-AR_RC=ar rc
 CC=gcc
+AR_RC=ar rc
+RANLIB=ranlib
 MKDIR_P=mkdir -p
 LN_SF=ln -sf
-RANLIB=ranlib
 CP=cp
-RM_F=/bin/rm -f
+RM_F=rm -f
 
 # where "make install" puts libpng16.a, libpng16.so*,
 # libpng16/png.h, libpng16/pngconf.h, and libpng16/pnglibconf.h
@@ -42,13 +42,12 @@ ZLIBINC=../zlib
 
 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 -O3 -funroll-loops # $(WARNMORE) -g
-
-LDFLAGS=-L. -Wl,-rpath,. -L$(ZLIBLIB) -Wl,-rpath,$(ZLIBLIB) -lpng16 -lz -lm
-LDFLAGS_A=-L$(ZLIBLIB) -Wl,-rpath,$(ZLIBLIB) libpng.a -lz -lm
+CFLAGS=-O3 -funroll-loops -Wall -Wextra -Wundef # $(WARNMORE) -g
+LDFLAGS=-L. -Wl,-rpath,. -L$(ZLIBLIB) -Wl,-rpath,$(ZLIBLIB) -lpng16 -lz -lm # -g
+LDFLAGS_A=-L$(ZLIBLIB) -Wl,-rpath,$(ZLIBLIB) libpng.a -lz -lm # -g
 
 INCPATH=$(prefix)/include
 LIBPATH=$(exec_prefix)/lib