/usr/include : Directory for standard include files.

This is where all of the system's general-use include files for the C programming language should be placed.

The following directories, or symbolic links to directories, must be in /usr/include, if the corresponding subsystem is installed:

Directory	Description
bsd	BSD compatibility include files (optional)

The symbolic link /usr/include/X11 must link to /usr/X11R6/include/X11 if the latter exists.