depends: qt: Fix C{,XX} pickup
Summary:
QT LOVES to do things its own way and simply ignore all environment variables. Even listing QMAKE_CC=... in ./configure flags does't work reliably. This is why we gotta literally edit their config files for them. Thanks QT.
Backport of core PR17466.
Depends on D5656.
Test Plan: Run the OSX Gitian build.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5657