Improve compatibility for old formula

Allow compatibility to be disabled with --no-compat or HOMEBREW_NO_COMPAT environment variable.

Library/Homebrew/compat/brewkit.rb

@@ -0,0 +1,3 @@
+# here so that formula from the repo that are very old can still be installed
+
+require 'formula'

Library/Homebrew/compat/compatibility.rb

@@ -76,5 +76,7 @@ class Formula
   def fails_with_llvm msg=nil, data=nil
     handle_llvm_failure FailsWithLLVM.new(msg, data)
   end
-
+end
+
+class UnidentifiedFormula < Formula
 end

Library/Homebrew/global.rb

@@ -3,7 +3,6 @@ require 'extend/ARGV'
 require 'extend/string'
 require 'utils'
 require 'exceptions'
-require 'compatibility'
 
 ARGV.extend(HomebrewArgvExtension)
 
@@ -58,3 +57,8 @@ end
 
 FORMULA_META_FILES = %w[README README.md ChangeLog COPYING LICENSE LICENCE COPYRIGHT AUTHORS]
 PLEASE_REPORT_BUG = "#{Tty.white}Please report this bug: #{Tty.em}https://github.com/mxcl/homebrew/wiki/Checklist-before-filing-a-new-issue#{Tty.reset}"
+
+unless ARGV.include? "--no-compat" or ENV['HOMEBREW_NO_COMPAT']
+  $:.unshift(File.expand_path("#{__FILE__}/../compat"))
+  require 'compatibility'
+end