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.64sunu

@@ -1,6 +1,6 @@
 # makefile for libpng on Solaris 2.x with cc
 # Contributed by William L. Sebok, based on makefile.linux
-# Copyright (C) 2020-2022 Cosmin Truta
+# Copyright (C) 2020-2024 Cosmin Truta
 # Copyright (C) 2002, 2006, 2010-2014 Glenn Randers-Pehrson
 # Copyright (C) 1998 Greg Roelofs
 # Copyright (C) 1996, 1997 Andreas Dilger
@@ -19,9 +19,8 @@ LIBSOMAJ=$(LIBNAME).so.$(PNGMAJ)
 
 # Utilities:
 CC=cc
-AR_RC=ar rc
+AR=ar
 RANLIB=echo
-MKDIR_P=mkdir -p
 LN_SF=ln -f -s
 RM_F=/bin/rm -f
 
@@ -41,7 +40,8 @@ WARNMORE=-Wwrite-strings -Wpointer-arith -Wshadow \
 	-Wmissing-declarations -Wtraditional -Wcast-align \
 	-Wstrict-prototypes -Wmissing-prototypes # -Wconversion
 CPPFLAGS=-I$(ZLIBINC) # -DPNG_DEBUG=5
-CFLAGS= $(SUN_CC_FLAGS) # $(WARNMORE) -g
+CFLAGS=$(SUN_CC_FLAGS) # $(WARNMORE) -g
+ARFLAGS=rc
 LDFLAGS=-L. -R. $(SUN_LD_FLAGS) -L$(ZLIBLIB) -R$(ZLIBLIB) -lpng16 -lz -lm
 
 OBJS = png.o pngerror.o pngget.o pngmem.o pngpread.o \
@@ -65,7 +65,7 @@ DELETE = $(RM_F)
 DFNFLAGS = $(DEFS) $(CPPFLAGS)
 
 libpng.a: $(OBJS)
-	$(AR_RC) $@ $(OBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS)
 	$(RANLIB) $@
 
 $(LIBSO): $(LIBSOMAJ)