Find the header or source file corresponding to this file.
Being on a `#include' line pulls in that file.
If optional IN-OTHER-WINDOW is non-nil, find the file in the other window.
If optional IGNORE-INCLUDE is non-nil, ignore being on `#include' lines.
Variables of interest include:
 - `ff-case-fold-search'
   Non-nil means ignore cases in matches (see `case-fold-search').
   If you have extensions in different cases, you will want this to be nil.
 - `ff-always-in-other-window'
   If non-nil, always open the other file in another window, unless an
   argument is given to `ff-find-other-file'.
 - `ff-ignore-include'
   If non-nil, ignores #include lines.
 - `ff-always-try-to-create'
   If non-nil, always attempt to create the other file if it was not found.
 - `ff-quiet-mode'
   If non-nil, traces which directories are being searched.
 - `ff-special-constructs'
   A list of regular expressions specifying how to recognize special
   constructs such as include files etc, and an associated method for
   extracting the filename from that construct.
 - `ff-other-file-alist'
   Alist of extensions to find given the current file's extension.
 - `ff-search-directories'
   List of directories searched through with each extension specified in
   `ff-other-file-alist' that matches this file's extension.
 - `ff-pre-find-hook'
   List of functions to be called before the search for the file starts.
 - `ff-pre-load-hook'
   List of functions to be called before the other file is loaded.
 - `ff-post-load-hook'
   List of functions to be called after the other file is loaded.
 - `ff-not-found-hook'
   List of functions to be called if the other file could not be found.
 - `ff-file-created-hook'
   List of functions to be called if the other file has been created.
(fn &optional IN-OTHER-WINDOW IGNORE-INCLUDE)