autoconf: force use of system perl

If a non-system perl is present when autoconf is built and subsequently
removed, autoconf will break. Force the use of the system perl to
prevent this.

Formula/autoconf.rb

@@ -11,6 +11,8 @@ class Autoconf < Formula
   end
 
   def install
+    ENV['PERL'] = '/usr/bin/perl'
+
     # force autoreconf to look for and use our glibtoolize
     inreplace 'bin/autoreconf.in', 'libtoolize', 'glibtoolize'
     system "./configure", "--prefix=#{prefix}"