Does using CMAKE_OSX_DEPLOYMENT_TARGET=10.15 suffice? The problem is that these older macOS versions EOL pretty rapidly and newer Xcode versions are barred from them. I think just using a newer SDK on the older release (and the deployment target setting) may be sufficient as well.
There’s not really a way to edit these paths reliably; they’re just strings to CMake. The more proper way would be for them all to be targets and to be re-found at use time so that they can adapt to whatever the ambient environment is as necessary.