It is telling you that /usr/include/IL/... doesn't exist.
If you use the -I option you need to specify the path to the directory referenced (IL) in this case. Not the directory itself. If you have IL installed in your home directory then the path would be
/home/install, where /home/install/IL/IL.h would be the full path.
Alternatively, you can also use relative paths - If you have the directory IL in your working directory then you can use -I . (current directory).
The default operation for #include <something.h> is to include the file /usr/include/somehting.h. If it is <something/something.h> then it is to include the file /usr/include/something/something.h. To specify other directories you have to specify the path to replace the "/usr/include"
with; and that option is -I for the compilers.