[DEPENDS] Make all the x* packages 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 build-linux # This target should be benefit the most from this
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4861