info-lookup-file-name-alist

info-lookup-file-name-alist is a variable defined in `info-look.el'.
Its value is

(("\\`ac\\(local\\|site\\|include\\)\\.m4\\'" . autoconf-mode))

Documentation:
Alist of file names handled specially.
List elements are cons cells of the form

(REGEXP . MODE)

If a file name matches REGEXP, then use help mode MODE instead of the
buffer's major mode.

You can customize this variable.