Flatpak

Flathub’s builders don’t allow internet access during the build; everything that goes into the build must be specified in the manifest. This means meson submodules won’t work. Instead, you need to install blueprint-compiler as a module in your flatpak manifest:

{
  "name": "blueprint-compiler",
  "buildsystem": "meson",
  "cleanup": ["*"],
  "sources": [
    {
      "type": "git",
      "url": "https://gitlab.gnome.org/jwestman/blueprint-compiler",
      "tag": "v0.12.0"
    }
  ]
}

You can keep the submodule configuration–Meson will ignore it if blueprint-compiler is already installed.