make plugin make-parameters not handled correctly

[Alan Pope 🍺🐧🐱 🦄]

Create the following yaml, based on https://github.com/snapcore/snapcraft/blob/master/demos/git/snapcraft.yaml

Note the DESTDIR make-parameters setting.

name: git
version: 2.8.0
summary: Git is a free and open source distributed version control system.
description: This example is not really production quality
confinement: strict

apps:
  server:
    command: bin/git

parts:
  git:
    plugin: make
    source: https://github.com/git/git
    source-type: git
    make-parameters:
      - DESTDIR=foo
    build-packages: [gettext, libssl-dev, libcurl4-openssl-dev, libexpat1-dev]

Output as below when run:-

http://paste.ubuntu.com/18634731/

note DESTDIR passed to make in line 20, and passed to make install in line 698. However on line 698 it _also_ passes another DESTDIR, overriding the one in the yaml. I would expect parameters set in yaml to supersede those defaults.