``#include_next`` is a gcc-specific language extension that alters the search path for the specified header file by starting the search from the header file directory _after_ the one in which the directive was encountered. It also ignores the distinction between ``"file"`` and ``<file>``. It is typically used when you have two (probably related) header files with the same name, although there is nothing in the extension to enforce or limit the use to same-name files.
Use of this extension can be tricky to get right, and is almost never justified. Instead, you should use an absolute path in the ``#include`` statement or rename one of the files.
