Preliminary support for M1 builds

NOTE: I don't have an M1 Mac, so the only testing I can do is with
a non-`/usr/local` Intel Homebrew installation. This should take
care of "`cc` shim forbids `/usr/local` if Homebrew not installed
under `/usr/local`", which is the default setup for M1 Homebrew.

Formula/avfs-mac.rb

@@ -20,6 +20,7 @@ class AvfsMac < Formula
   depends_on "xz"
 
   def install
+    setup_fuse
     args = %W[
       --prefix=#{prefix}
       --disable-debug