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.