This option adds the specified directory, or comma-separated list of directories, to the list of places that are searched to find included files.
If you specify more than one directory, the directories are
searched in the same order as the
-I options specifying
is a comma-separated list of directories to be searched for included files.
At least one directory must be specified.
When specifying multiple directories, do not include spaces between commas and directory names in the list.