Add dev_tools_path to PATH if not in PATH already

This prevents what are likely a whole slew of bugs.

Library/Homebrew/build.rb

@@ -23,6 +23,9 @@ at_exit do
     ENV.setup_build_environment
     # we must do this or tools like pkg-config won't get found by configure scripts etc.
     ENV.prepend 'PATH', "#{HOMEBREW_PREFIX}/bin", ':' unless ORIGINAL_PATHS.include? "#{HOMEBREW_PREFIX}/bin"
+    # this is a safety measure for Xcode 4.3 which started not installing
+    # dev tools into /usr/bin as a default
+    ENV.prepend 'PATH', MacOS.dev_tools_path, ':' unless ORIGINAL_PATHS.include? MacOS.dev_tools_path
 
     install(Formula.factory($0))
   rescue Exception => e