[DEPENDS] Make the qt package build parallel
[DEPENDS] Make the qt package build parallel

Summary:
As per title. The install phase is left intact as there is almost
nothing to gain here.

Test Plan:
  cd depends
  make

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4867

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