Add support for PHP 5.6 pre-releases.

* Look for downloads in Tyrael's repository.
* Support --enable-phpdbg.
* Automatically add --enable-phpdbg-debug to $configoptions
  when the "debug" flag is present in the version string
  and --enable-phpdbg is present in $configoptions.
* Create a versioned symlink for phpdbg after installation.