Moritz Angermann authored Jul 28, 2017 and Ben Gamari committed Jul 28, 2017

This allows to customize the location where binary distributions are
placed with `make binary-dist`.

E.g. using:
```
BINARY_DIST_DIR=/path/to/bindists make binary-dist
```
will place binary dists outside of the source tree into the given
folder.

This change falls back to ".", which is the old behaviour.

Test Plan: build binary-dist

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3798