Writing buildlink files: optional dependencies?

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

When creating a buildlink3.mk for a package foo that exposes libfoo, if 
libfoo depends on libbar (from package bar), my understanding is that I 
should include ../../whatever/bar/buildlink3.mk in foo's buildlink3.mk.

But how do I handle the case where foo only depends on bar when built with 
the "bar" option? Do I consult PKG_OPTIONS.foo? If yes, how do I make sure 
that the newly created buildlink3.mk doesn't run out of sync with the 
options.mk file (of foo)? The next version of foo built with the "bar" 
option could depend on bar-ng.