3.81 doesn't understand the ‘define foo =’ syntax, which was added in 3.82. So use ‘define foo’ instead.
This is a set of helper Makefiles for doing non-recursive builds with
GNU Make. The canonical source can be found at
https://github.com/edolstra/make-rules. You should copy the files
into the mk subdirectory of your project.
TODO: write more documentation.