New review comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/28972#discussion_r580372372

Comment:
If possible, it would be nicer to use `make_build_args` instead of `do_build`. You'd need to append to `make_build_args` in `pre_build`, though, since `pkg-config` won't be available before that.

Btw, `JPEG_DIR=""` and `JPEG_DIR=` do the same thing :)