For PR automake/375 (Debian Bug #168671):

For PR automake/375 (Debian Bug #168671):
* automake.in (scan_texinfo_file): Erase all known index files, no
matter whether they are used at the top-level or not, because we
can never know if they are used in included files.  The remaining
macros we recognize at the top-level of a Texi document are
@defindex, @defcodeindex, @synindex, and @syncodeindex.
* tests/texinfo18.test: New file.
* tests/Makefile.am (TESTS): Add texinfo18.test.
Reported by Simon Richter.