Allow the Rakefile to function even when cxx_dependency_map.rb does not exist
Allow the Rakefile to function even when cxx_dependency_map.rb does not exist

So that when there is a merge conflict on this file, we can just 'rm'
it and run 'rake dependency_map' instead of editing it manually with
the editor.

phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger