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.emcc

@@ -1,5 +1,5 @@
 # makefile for libpng using emscripten
-# Copyright (C) 2000, 2014, 2019-2022 Cosmin Truta
+# Copyright (C) 2000, 2014, 2019-2024 Cosmin Truta
 # Copyright (C) 2021 Kirk Roerig
 # Copyright (C) 2008, 2014 Glenn Randers-Pehrson
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
@@ -15,13 +15,14 @@ ZLIBLIB = ../zlib
 # Compiler, linker, lib and other tools
 CC = emcc
 LD = $(CC)
-AR_RC = emar rcs
+AR = emar
 RANLIB = emranlib
 CP = cp
 RM_F = rm -f
 
 CPPFLAGS = -I$(ZLIBINC) # -DPNG_DEBUG=5
 CFLAGS = -O2 -Wall -Wextra -Wundef
+ARFLAGS = rc
 LDFLAGS = -L$(ZLIBLIB)
 PNGTEST_LDFLAGS = --preload-file=pngtest.png
 LIBS = -lz -lm
@@ -51,7 +52,7 @@ shared:
 	@false
 
 libpng.a: $(OBJS)
-	$(AR_RC) $@ $(OBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS)
 	$(RANLIB) $@
 
 test: pngtest