This removes support of separate qmake versions in one port: as we
discovered, there are no ports actually needing this; strangers like
print/poppler don't use qmake in build.
This should be transparent to current ports. But expect more tweaks there:
for now, qt?.port.mk forces qmake.port.mk inclusion, but that will be
reworked to a more common scheme.
Same idea from (at least) espie@ and me; also, espie@ agrees on the plan.
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