wayfire-plugin meson libdir can be incorrect (Ubuntu 24.04)

Wayfire version

0.12 (latest git)

GPU / Driver

Not relevant

Describe the bug

I'm trying out the new wayfire-plugin command with my plugins, but ran into some problems. Specifically, I'm trying to follow the instructions here to adjust the build of a plugin.

To Reproduce

Steps to reproduce the behavior:

1. Write a simple plugin that uses meson
2. Use the recommended command to install, e.g.

scale_ipc_filter_lib = shared_module('scale_ipc_filter', sources,
    dependencies: [wayfire],
    install: true,
    install_dir: get_option('prefix') / get_option('libdir') / 'wayfire',
    cpp_args: ['-Wno-unused-parameter', '-Wno-format-security','-DWAYFIRE_PLUGIN'],
    link_args: '-rdynamic')

3. Plugin gets installed under: ~/.local/share/wayfire/plugin-manager/install/lib/x86_64-linux-gnu/wayfire/
4. Wayfire cannot load it as it is looking for plugins under ~/.local/share/wayfire/plugin-manager/install/lib/wayfire -- see here

Note: the plugin shows up as correctly installed with wayfire-plugin:

wayfire-plugin list
wayfire-scale-ipc: 0.12.0 ABI 20260801

Expected behavior

Wayfire can find and load the plugin.

Apparently, on Ubuntu (and I assume Debian as well), get_option('libdir') expands to lib/x86_64-linux-gnu by default. Everything works as expected if I change the meson install command to include:

install_dir: get_option('prefix') / 'lib' / 'wayfire',

I suggest to either:

• update the doc to suggest using 'lib' as that is where Wayfire will look for (and there does not seems to be a downside of a hardcoded path, since this is a directory only used by Wayfire)
• change the path discovery to use the "libdir" that meson does via config.h (that now only contains a full path to system plugins)

Let me know which solution is preferred, and I can create a PR.

Start with docs/wayfire-plugin-author.md and src/core/plugin-loader.cpp around line 329, then reproduce the Meson installation on Ubuntu 24.04 using the command shown. Determine whether the documentation or plugin path discovery should change, and verify that a plugin installed with wayfire-plugin can be found and loaded.