I can't find any way to fix this.

Doesn't matter, I compiled it fine many times. Ignore it.

It won't let me compile it without it. The build fails

I'm guessing you don't have a build.properties file.
Follow these steps: https://github.com/pahimar/Equivalent-Exchange-3#setup-ee3-some-tested-for-linux-tested-fully-for-windows (specifically Step 1)

EDIT: Yeah, I'm thinking the build will fail without setting ${dir.development}.

Yup, the problem is an incorrectly configured build.properties. All the variables in the ant script need values specified in your own build.properties