Revert "qt4: try enableParallelBuilding again"
Revert "qt4: try enableParallelBuilding again"

This reverts commit e8dc6258a1b5b11846fecf4a00c56a308dd22527.

It's unfortunate, but there still are occurrences of the missing ui_qtgradientview.h
on Hydra: http://hydra.nixos.org/build/12625390/nixlog/1/tail-reload

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