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.std

@@ -1,5 +1,5 @@
 # makefile for libpng
-# Copyright (C) 2020-2022 Cosmin Truta
+# Copyright (C) 2020-2024 Cosmin Truta
 # Copyright (C) 2002, 2006, 2014 Glenn Randers-Pehrson
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
 #
@@ -16,9 +16,8 @@ ZLIBINC=../zlib
 CC = cc
 CPP = $(CC) -E
 LD = $(CC)
-AR_RC = ar rc
+AR = ar
 RANLIB = ranlib
-MKDIR_P = mkdir
 MV_F = mv -f
 RM_F = rm -f
 AWK = awk
@@ -29,6 +28,7 @@ DFNFLAGS = # DFNFLAGS contains -D options to use in the libpng build
 DFA_EXTRA = # extra files that can be used to control configuration
 CPPFLAGS = -I$(ZLIBINC) $(NOHWOPT) # -DPNG_DEBUG=5
 CFLAGS = -O # -g
+ARFLAGS = rc
 LDFLAGS = -L$(ZLIBLIB) # -g
 LIBS = -lz -lm
 
@@ -66,7 +66,7 @@ pnglibconf.dfn: scripts/pnglibconf.dfa scripts/options.awk pngconf.h pngusr.dfa
 	$(MV_F) pnglibconf.tmp $@
 
 libpng.a: $(OBJS)
-	$(AR_RC) $@ $(OBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS)
 	$(RANLIB) $@
 
 pngtest: pngtest.o libpng.a