Add support for user specified Makefile targets

[ILIV]

This is something I ended up wishing was possible when wrestling with issue documented in #1636238.

PostgreSQL contrib modules can be installed in two recommended ways:

1. cd src/contrib/; make; make install (which currently doesn't work; please, see for #1636238 details)
2. build world target when compiling all of the PostgreSQL. i.e. make world, make install.

However, there doesn't seem to be a way to tell the make plugin which Makefile target a user would like to build, or install.

[abdullahc (sneetsher)]

Check the below doc page, that's already implemented:

https://snapcraft.io/docs/reference/plugins/common

Example:

parts:
  part_one:
    plugin: make
    build: |
      make build
    install: |
      make install_api
      make install_app

Please, close the report if that's exactly what you need.