On macOS, `sw_vers` may print to `stderr` based on environment variables
like `MallocGuardEdges=1`. Add a test to verify that we do not capture
that as part of the information it prints to `stdout`.
Issue: #26466
When using the Visual Studio generator for VS 10 or above,
offer this key to get the location of the MSBuild command
before the first `project()` or `enable_language()` command
has finished running.
This will be needed only by one of our own modules, so leave it
undocumented for now.
Brad King