24.4. .include search path: -I path

Use this option to add a path to the list of directories the assembler searches for files specified in .include directives (see Section 29.29: Include.). You may use -I as many times as necessary to include a variety of paths. The current working directory is always searched first; after that, the assembler searches any -I directories in the same order as they were specified (left to right) on the command line.