Summary:
Given:
  a Makefile with a non-PHONY target called `target`
If:
  after running `make target`, a file exists with the same name as `target`
  (it was either already there, or it was created by running `make target`)
And:
  `target` has no dependencies, such as `clean`, that modify or delete that
  file
Then:
  subsequent invocations of `make target` will not have any effect.

Solution: make `target` PHONY.

BAD:
  ```
  foo:
          ...
          ./foo
  ```

BETTER:
  ```
  foo:
          ...
          ./foo
  .PHONY: foo
  ```

Reviewers: austin

Reviewed By: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D670