Retrieve ruby core source files.

This can be used by any C extension gem that needs Ruby core headers (*.h and *.inc)

Usage:

  Ruby_core_source::create_makefile_with_core(hdr_check_proc, gem_name)
takes the place of
  create_makefile(gem_name)
in your extconf.rb

This will:
  If hdr_check_proc is true, call create_makefile(gem_name)
  Else: if core headers already exist, and hdr_check_proc is true, call create_makefile(gem_name)
  Else: fetch and extract the core headers, check hdr_check_proc, and call create_makefile(gem_name)
  Else: return false

Note that for Ruby preview releases, the corresponding RUBY_REVISION needs to be maintained in
http://cloud.github.com/downloads/mark-moseley/ruby_core_source/preview_revision.yml

Example use in extconf.rb:

require 'ruby_core_source'
hdrs = proc { have_header("vm_core.h") and have_header("iseq.h") }
dir_config("ruby") # allow user to pass in non-standard core include directory
if !Ruby_core_source::create_makefile_with_core(hdrs, "foo")
  # error
  exit(1)
end