ree - explicitly locate readline

Fixes Homebrew/homebrew#2579

Formula/ruby-enterprise-edition.rb

@@ -14,9 +14,13 @@ class RubyEnterpriseEdition <Formula
   end
 
   def install
+    readline = Formula.factory('readline').prefix
+
     fails_with_llvm "fails with LLVM"
     args = ['./installer', "--auto", prefix, '--no-tcmalloc']
     args << '-c' << '--enable-shared' if ARGV.include? '--enable-shared'
+    # Configure will complain that this is an unknown option, but it is actually OK
+    args << '-c' << "--with-readline-dir=#{readline}"
     system *args
   end