automake: force use of system perl

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

Formula/automake.rb

@@ -14,6 +14,8 @@ class Automake < Formula
   end
 
   def install
+    ENV['PERL'] = '/usr/bin/perl'
+
     system "./configure", "--prefix=#{prefix}"
     system "make install"