depends macOS: point --sysroot to SDK

Summary:
point --sysroot to SDK in order to support building on macOS Catalina

Backport of core PR17118.

Test Plan:
On macOS Catalina:

cd depends
make build-osx NO_PROTOBUF=1 # building protobuf fails, needs investigation

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, rex4539

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4816