port dependencies with port options

[Vitaly Magerya]

Chris Inacio wrote:
> 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?

One way to do this is to create a separate port net/libfixbuf-spread
that either installs separate libfixbuf-spread.so or just conflicts with
net/libfixbuf, and then make net-mgmt/yaf depend on that if SPREAD
option is on.