should a spec file tell about --build-in-place?

[lnussel]

When sources of a package as well as the spec file are in git (example), the --build-in-place option for rpmbuild is quite convenient to build the package directly from the checkout directory. How does one know whether a spec file is meant for in-place building though? Esp when that someone is an automated build system calling rpmbuild?
Is it expected that the surrounding build system adds some extra metadata that tells it to use in-place building?
Should spec files have some tag that is understood by rpmbuild so it switches to in-place building automatically?