I have been building the client using VS2013 and also the latest Linux gcc (GitHub)
Recently was able to build the milkyway app for windows on Linux using mingw cross compiler (githhub)
Also built TBar's "special seti" source on Linux using latest gcc and CUDA libs (found zip at forum)

Was looking at building a windows version of that seti app and found a problem:

1>CUDACOMPILE : nvcc warning : nvcc support for Microsoft Visual Studio 2013 and earlier has been deprecated and is no longer being maintained
1>  support for this version of Microsoft Visual Studio has been deprecated! Only the versions between 2015 and 2019 (inclusive) are supported!

This is not a problem for the client as it does not run any CUDA code. However, the app clearly needs to be built with CUDA and I am guessing the newer libraries from NVidia might not be linkable with object code build by VS2013. That seti app uses source code from the client, especially include files and using VS2017 will require mods to the sources. I can try building the seti app using VS2017 and was wondering if there is any active work in making the client compatible with VS2015 or later? Possibly the seti app is best built with the mingw cross compiler instead of any MS product.

The section of code you need for this purpose isn't the client code as a whole.

It's one specific library, known as the 'API' (Application Programming Interface), which handles the communications between the application and the client (stop, start, how far, how long, etc.)

Search the BOINC docs for API - I think it's designed to be built separately in the build tool of your choice, and linked to the science code in the final stage. It should be a relatively simple build with no user interface components, so you should probably expect to use the scientific compilers, rather than the heavyweight Visual Studio.