Thanks Thomas, that was super helpful. I've created GH-20034 to add in the compile-time arguments on POSIX systems at least, do you mind having a look?

For the moment I have made it non-configurable on Windows, but I think the right thing to do is to add an argument to PCbuild\build.bat. Hopefully Steve can point me in the right direction for mapping that argument (or something else) to a new config variable in sysconfig.