listing the WITH_* variables for each package

[Simon Barner]

> One thing I always find annoying is the necessity to dig through the 
> Makefiles to look at all the WITH_* variables that are available when 
> compiling a port.  I think there are a couple easy options to do 
> something better here.
> 
> 1. Have a suggestion in the porter's handbook to list them in pkg-descr. 
>  Along with each WITH variable would be a one line description.  This 
> would be very helpful and not much extra work.
> 
> 2. Create another (optional) file such as pkg-options in each port.  It 
> would simply be a listing of the available WITH options and a one line 
> description.  Experimental or developer-only options could be left out 
> or marked differently.  This would also open additional automated build 
> possibilities such as doing test builds with all possible subsets of the 
> WITH variables (obviously this would be time consuming, but wouldn't 
> need to be done very often).

3. A `make options' target similar to `make maintainer'