How do I configure things so that system and library files are included in the project?

[pedz]

Forgive me: I'm not sure if this is a projectile question or an lsp-mode question.

I have a project that is using projectile, lsp-mode, ccls, and others that lives in ~/projects/this-project. This is a project for a ESP32 (much like Arduino) board so the header files for the system and common libraries are in a directory called ~/.platformio.

Lets say that foo.c is in my project and lives in the project's root directory. It includes header.h which lives in ~/.platformio.

When I load up foo.c, it automatically goes into lsp-mode and things work correctly. In particular, the paths of the include files are lit up and I can put the cursor within the path, hit M-. and Emacs finds the file.

But header.h does not go into lsp-mode. I think I can probably fix that but it is key here. Fortunately when I enter lsp-mode, I get a prompt to tell it where the root of the project is. I hit I (capital i) and give it the path back to ~/projects/this-project. This is coming from lsp--find-root-interactively.

At this point, things work as I want them to mostly. lsp-mode still says it can't find some header files which it should be able to given what is in the .ccls. So, even this has limited utility but the lsp-describe-session looks closer to what I think I want.

There is an elisp routine called project-external-roots which eglot and xref use but none of the other packages I have do. I'm wondering if either projectile or lsp-mode needs to use it (or if I can somehow hack it in).