Creating port with options

[Alejandro Pulver]

Hello,

I am creating a port of the Core War system pMARS (the existent port is
version 0.8, lastest is 0.9.2), and I will allow the user to choose the
GUI and features. I have some questions:

How can I "document" the options for the user?

So that typing "make" will not do anything unless it has the required
options (or it will use the defaults but first shows the available
options). Will be possible for the user to store the configuration
somwehere? Wich is the best way to show the user the options (before
the building process)?

P.S.: how do ports show the sysinstall-like dialog to choose options?

Thanks and Best Regards,
Ale