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.aix

@@ -1,5 +1,5 @@
 # makefile for libpng using gcc (generic, static library)
-# Copyright (C) 2000, 2022 Cosmin Truta
+# Copyright (C) 2000, 2020-2024 Cosmin Truta
 # Copyright (C) 2002, 2006-2009, 2014 Glenn Randers-Pehrson
 # Copyright (C) 2000 Marc O. Gloor (AIX support added, from makefile.gcc)
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
@@ -15,9 +15,8 @@ ZLIBLIB = ../zlib
 # Compiler, linker, lib and other tools
 CC = gcc
 LD = $(CC)
-AR_RC = ar rcs
+AR = ar
 RANLIB = ranlib
-MKDIR_P = mkdir -p
 RM_F = rm -f
 
 LIBNAME = libpng16
@@ -26,6 +25,7 @@ PNGMAJ = 16
 WARNMORE =
 CPPFLAGS = -I$(ZLIBINC) # -DPNG_DEBUG=5
 CFLAGS = -O2 -Wall -Wextra -Wundef # $(WARNMORE) -g
+ARFLAGS = rc
 LDFLAGS = -L. -L$(ZLIBLIB) -lpng16 -lz -lm # -g
 
 # File lists
@@ -44,7 +44,7 @@ REMOVE = $(RM_F)
 DFNFLAGS = $(DEFS) $(CPPFLAGS)
 
 $(LIBNAME).a: $(OBJS)
-	$(AR_RC) $@ $(OBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS)
 	$(RANLIB) $@
 
 test: pngtest