build: Propagate well-known vars into depends

Summary:
This will makes the boost depends build follow the CC/CXX environment
variables (and allow the build with clang).

Backport of core PR18820.

Test Plan: Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7672