meson plugin builds to /usr/local by default

[Alan Pope 🍺🐧🐱 🦄]

The meson plugin appears to put builds by default in /usr/local. This doesn't seem like a sensible default, so most application developers will have to specify --prefix for /usr which seems more common and consistent.

[Cris Dywan (kalikiana)]

You'll probably want to add a check in snapcraft/plugins/meson.py _meson_run like "if '--prefix' not in self.options.meson_parameters" and then meson_command.extend() to add it only if it wasn't specified. And don't forget to add it to schema['properties'] at the top of the file.