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 them.
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.