Rationale:

• The ARM gcc compilers version > 7 are required to pass the symbol

check, but stretch provides version 6.

• CMake version >= 3.8 is required to use boost 1.64 with imported

targets (this is the current version from the depends/ subsystem), but
stretch provides 3.7.

• It seems to make the windows gitian build deterministic.

The instructions get updated accordingly. The debian VM has also been
bumped to Buster.

Depends on D4167.

Follow the instructions to setup the gitian build.
Note: I only tested the Debian over Debian path, Fedora and Vagrant are
untested.
Run all the gitian build twice.
Check the builds are successful and deterministic.
Check the binaries run successfully on their target platform.