Lookup always fails when header path is missing

[Ambrevar]

If I use a library which is not found in the paths set by c-eldoc-includes, all lookups fail, including those for which the libraries are found.

[nflath]

This is because this works by running the preprocessor, which will load all the includes, then parsing/scanning that. So if a header is missing from the path, this step will fail and we will be unable to parse.