in src\tools\msvc\Solution.pm the following test is not sufficient $self->{platform} = ($output =~ /^\/favor:<.+AMD64/m) ? 'x64' : 'Win32'; because the cl /? returns: Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x64 Copyright (C) Microsoft Corporation. All rights reserved.
I see no bug here, that expression is looking for the line: "/favor:<blend|AMD64|INTEL64|ATOM> select processor to optimize for, one of:", which is one of the first printed options.
To make sure this is working in your environment, you should see "Detected hardware platform: x64" when building.