Get rid of QT_PREFIX in favour of PREFIX

[tcberner]

Summary

I don't see a reason to have our own QT_PREFIX variable.
I think it is more sensible to use PREFIX.

Test Plan

• test [some] affected ports with poudriere -P
• exp-run

[tcberner]

I did not have the patience to wait for editors/libreoffice* to finish. The rest of the affected ports were fine.

[rakuco]

Looks good to me. The use of QT_PREFIX seems to date back to the days we had Qt 3 and Qt 4 in the tree (the latter still being worked on) and Qt3 installed into ${X11BASE}.

Make sure you add an entry to CHANGES too.

multimedia/phonon/pkg-plist

187 ↗

(On Diff #23024)

I think it makes sense to re-sort the plist now.

[AMDmi3]

This was broken, as PREFIX is used where LOCALBASE was and should have used.