conditionally including packages in buildlink3.mk based upon selected options

["Blair Sadewitz" <blair.sadewitz%gmail.com@localhost>]

I'm working on a libX11-1.1.1 package, and I have an option which
selects either phreads or xcb.

If xcb is selected, then another package needs to be included in
libX11's buildlink3.mk.  However, I cannot just blindly do this, as if
xcb is not used this will cause all sorts of problems.

What is the most elegant way to take care of this?
I have found many possible hints at solutions in the documentation,
but nothing concrete--and I'd like to learn the right way from the
get-go.

Thanks,

--Blair
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