Note that IIUC, for non-system includes to work, either - the filenames must be absolute, or - the compiler must be given -I arguments through org-babel-C-compiler. This variable can be set (e.g. to "gcc -I .") with file or directory-local variables. Should we promote this method in NEWS? A downside is that the user will be warned about the variable's value being potentially unsafe, and we can't really avoid that unless we throw a blanket :safe #'stringp on this defcustom.