port dependencies with port options

[Chris Inacio]

Hello ports wizards,

I wanted to add an option to multiple ports - that is easy.  But, those
ports have a dependency relationship, and I only want the last node in the
port dependency graph to build with that option if the requisite ports have
too.

In real terms:

net/spread <- net/libfixbuf <- net-mgmt/yaf

I added a SPREAD option to net/libfixbuf & to net-mgmt/yaf.  net-mgmt/yaf
can only build a Spread version if libfixbuf was built with a Spread
version.

Question 1)  How do you construct such that if a user goes into
net-mgmt/yaf chooses Spread and fixbuf isn't already installed, it builds
fixbuf with the spread option?

Question 2) How do you ensure that if fixbuf is already installed, it has
the Spread option enabled, or disallow/error the Yaf Spread option?

Thanks,
Chris Inacio