/usr/lib/GNUstep/Bundles/libgnustep-art-026.bundle/stamp.make is in gnustep-back0.26-art 0.26.2-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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OLD_GNUSTEP_STAMP_ASTRING = _GSBackend-