build: fix 'make help'

Summary:
This fixes the usage of `make help` in the top-level and subdirectories.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Test Plan: It worked now and didn't before.

Reviewers: hvr

Reviewed By: hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D692