Thanks for the pointers. I added two commands that might be useful in the default new projects. I didn't add more because for compilation it really depends on which platform the project is configured to build against (android, ios, web, windows, or macos). That should be sufficient for now.
BTW, the [wiki](https://wiki.geany.org/howtos/configurebuildmenu) is wrong about `[build_menu]`. The right settings is `[build-menu]` as documented in the [manual](https://www.geany.org/manual/current/index.html#custom-filetypes).