[CMAKE] Build the background dist image on native OSX
Summary:
D4381 introduced the generation of the background image to the dist
directory. This diff adds support for the image transformation using
tools available on OSX.
Depends on D4381.
Test Plan:
On OSX:
ninja osx-deploydir
Check the background file is correctly generated.
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D4382