madplay: update config.{guess,sub} for arm64

See: https://trac.macports.org/ticket/72126

audio/madplay/Portfile

@@ -13,7 +13,6 @@ long_description	MAD is a high-quality MPEG audio decoder.  \
 	All three audio layers (Layer I, Layer II, and Layer \
 	III a.k.a. MP3) are fully implemented.
 homepage	https://www.mars.org/home/rob/proj/mpeg/
-platforms	darwin
 master_sites	ftp://ftp.mars.org/pub/mpeg/
 checksums	sha1 29105eb27c1416aa33c8d1ab3404a8e5f0aecd3f
 
@@ -29,3 +28,12 @@ platform darwin i386 {
 	configure.cflags-append -msse
 	configure.args-append --build=${build_arch}-apple-darwin${os.major}
 }
+
+# Update config.guess and config.sub to recognise arm64.
+depends_build-append    port:automake
+post-patch {
+    set automake_dirs [glob -directory ${prefix}/share automake-*]
+    set automake_dir [lindex [lsort -command vercmp $automake_dirs] end]
+    copy -force ${automake_dir}/config.guess ${automake_dir}/config.sub \
+        ${worksrcpath}
+}