Qt extension install locations

[Michael Dickens]

I've been updating the qt* portfiles; last night it was for
qt4-mac[-devel], and today I'll hopefully check in those for qt3-mac &
related ports.  As I'm doing these changes, I'm also working over direct
dependencies to make sure they still function correctly -- and, changing
them as minimally as possible in most cases so that they work (for me). 
Most dependent ports do not create "Qt extensions" (e.g. plugins,
translations, extra includes or libraries), but instead make use of Qt
in some other way -- these get installed into ${prefix} since that makes
the most sense.  Then there are those ports, such as qca and qscintilla,
that are explicitly Qt extensions.  The former is currently installed in
${prefix} while the latter is installed into ${qt_dir}.  Both install
locations have their benefits and drawbacks; I don't know which way is
the better way to go.  Thoughts? - MLD