[DEPENDS] Make expat, fontconfig and freetype 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/D4863