After switching to meson, calling the `g-ir-scanner` command with the
`--warn-all` parameter in the generated `build.ninja` file will display these warnings:

```
Warning: MateMenu: symbol="DesktopEntryType": unknown namespace for identifier "DesktopEntryType"
Warning: MateMenu: symbol='desktop_entry_new': Unknown namespace for symbol 'desktop_entry_new'
Warning: MateMenu: symbol='EntryDirectory': Unknown namespace for identifier 'EntryDirectory'
```

The reason for these warnings is that we use `MateMenu` as `identifier_prefix`
and `matemenu` as `symbol_prefix`, but the symbols that report warnings do not
start with these prefixes.