smpeg2: fix build on arm64 linux

Author: cho-m

Formula/s/smpeg2.rb

@@ -41,10 +41,12 @@ class Smpeg2 < Formula
   patch :DATA
 
   def install
+    args = ["--with-sdl-prefix=#{Formula["sdl2"].opt_prefix}", "--disable-sdltest"]
+    # Help old config scripts identify arm64 linux
+    args << "--build=aarch64-unknown-linux-gnu" if OS.linux? && Hardware::CPU.arm? && Hardware::CPU.is_64_bit?
+
     system "./autogen.sh"
-    system "./configure", "--with-sdl-prefix=#{Formula["sdl2"].opt_prefix}",
-                          "--disable-sdltest",
-                          *std_configure_args
+    system "./configure", *args, *std_configure_args
     system "make"
     system "make", "install"