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.sgi

@@ -1,5 +1,5 @@
 # makefile for libpng.a and libpng16.so, SGI IRIX with 'cc'
-# Copyright (C) 2020-2022 Cosmin Truta
+# Copyright (C) 2020-2024 Cosmin Truta
 # Copyright (C) 2001-2002, 2006, 2007, 2010-2014 Glenn Randers-Pehrson
 # Copyright (C) 1995 Guy Eric Schalnat, Group 42, Inc.
 #
@@ -17,9 +17,8 @@ LIBSOMAJ=$(LIBNAME).so.$(PNGMAJ)
 
 # Utilities:
 CC=cc
-AR_RC=ar rc
+AR=ar
 RANLIB=echo
-MKDIR_P=mkdir -p
 LN_SF=ln -sf
 CP=cp
 RM_F=/bin/rm -f
@@ -41,6 +40,7 @@ WARNMORE=-fullwarn
 CPPFLAGS=-I$(ZLIBINC) # -DPNG_DEBUG=5
 #CFLAGS= $(ABI) -O $(WARNMORE) -KPIC # -g
 CFLAGS=$(ABI) -O $(WARNMORE) # -g
+ARFLAGS=rc
 LDFLAGS_A=$(ABI) -L. -L$(ZLIBLIB) -lpng16 -lz -lm # -g
 LDFLAGS=$(ABI) -L. -L$(ZLIBLIB) -lpng -lz -lm # -g
 LDSHARED=cc $(ABI) -shared -soname $(LIBSOMAJ) \
@@ -64,7 +64,7 @@ pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
 	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 libpng.a: $(OBJS)
-	$(AR_RC) $@ $(OBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS)
 	$(RANLIB) $@
 
 $(LIBSO): $(LIBSOMAJ)