depend on feature (was: Apache dependence)

[Sergey Matveychuk]

Kris Kennaway wrote:
> If you don't override the default value the situation is no different
> to now.  It probably won't actually install a second version of
> apache, because the *_DEPENDS should be on a file that is common to
> all versions of apache, but it might record the wrong package
> dependency information when registering the port installation.

We can implement depending on a FEATURE (common idea of lev at freebsd.org 
concretized by me):
Some port climes itself as feature provider by setting FEATURE variable.
E.g. FEATURE=apache
This feature save with port or separately in PKG_DBDIR.

A port depend on this feature claims it with *_DEPENDS.
E.g. BUILD_DEPENDS=apache
We can recognize it with ":dir" part absent. The port system looks for 
this feature, found appropriated port and records dependency information 
there.

Merits is obvious.
Fully backward compatible.
Cost: need patch pkg_install and bsd.port.mk.

What do you think?

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Sem.