To make clangd-backed lsp recognize your local dependencies, you may use JSON Compilation Database to specify where to find includes.
For a smaller projects indeed, simpler compile_flags.txt format exists:
-I./build/external/raylib-master/src/
-I./build/external/raylib-master/src/external/
-I./build/external/raygui/
-L./build/external/raylib-master/src/
-DRAYLIB_STATIC
-DPLATFORM_DESKTOP
-lraylib
-lGL
-lpthread
-lm
-ldl
--std=c99
-Wall
-Wextra
The example above helps me working on the raylib-based project in neovim.