Add the given directory to the list of directories searched for
compiled interface files (.cmi), compiled object code files
(.cmx), and libraries (.cmxa). By default, the current directory
is searched first, then the standard library directory. Directories
added with -I are searched after the current directory, in the order
in which they were given on the command line, but before the standard
library directory.
If the given directory starts with +, it is taken relative to the
standard library directory. For instance, -I +labltk adds the
subdirectory labltk of the standard library to the search path.