Using Lua macros in an RPM specfile

I've found using Lua macros in rpm spec files to be pretty useful. I haven't found many examples of their use online, so here's how I ended up using them.

I had a situation where I needed to make a subpackage from all the files in a number of different subdirectories. The structure of the files in the subdirectories was fixed, but the number of subdirectories could change over time, and I didn't want to have to update the spec file each time a new subdirectory was added or removed.