build: Update the makefiles for the benefit of cross-compilation

Split AR_RC into AR and ARFLAGS. The variables AR and ARFLAGS are
de-facto standards (like CC and CFLAGS, LD and LDFLAGS, etc.) that
may be overridden when running make. Moreover, configuring CC, LD,
AR, etc., to point to a cross-platform compiler, linker, librarian,
etc., is a de-facto standard practice as well.

Also remove the MKDIR_P variable definitions from all makefiles.
They've been leftovers from the removal of the "install*" targets.

scripts/makefile.mips

@@ -1,5 +1,5 @@
 # makefile for libpng
-# Copyright (C) 2020-2022 Cosmin Truta
+# Copyright (C) 2020-2024 Cosmin Truta
 # Copyright (C) 1998-2014 Glenn Randers-Pehrson
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
 #
@@ -8,15 +8,15 @@
 # and license in png.h
 
 CC=cc
+AR=ar
+#RANLIB=ranlib
+RANLIB=echo
 CPPFLAGS=-I../zlib -DSYSV -Dmips
 CFLAGS=-O -systype sysv -w
 #CFLAGS=-O
+ARFLAGS=rc
 LDFLAGS=-L. -L../zlib/ -lpng -lz -lm
 
-AR_RC=ar rc
-#RANLIB=ranlib
-RANLIB=echo
-
 # Pre-built configuration
 # See scripts/pnglibconf.mak for more options
 PNGLIBCONF_H_PREBUILT = scripts/pnglibconf.h.prebuilt
@@ -34,7 +34,7 @@ pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
 	cp $(PNGLIBCONF_H_PREBUILT) $@
 
 libpng.a: $(OBJS)
-	$(AR_RC) $@ $(OBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS)
 	$(RANLIB) $@
 
 pngtest: pngtest.o libpng.a