Make it easier to add extra configure flags

Currently to add extra configure flags to an autotools module, as far as I can tell you have to do something like this in order to avoid overriding autogen and the default configure flags:

config:
  configure: %{conf-cmd} %{conf-args} --enable-gtk-doc

It would be nice if there were a customization point to just add extra arguments:

config:
  extra-configure-args: --enable-gtk-doc