configure.ac: Reject all texi2html versions except 1.82.

configure.ac: Reject all texi2html versions except 1.82.

LilyPond's customization file for texi2html is tied to that version, and
updating to recent releases of GNU Texinfo is a non-trivial, major
undertaking because the API has completely changed.

Also contains the following commits:

* aclocal.m4: STEPMAKE_PROGS: Add test for maximum version.


* aclocal.m4: STEPMAKE_CHECK_VERSION: Add test for maximum version.

  This replaces the optional $4 argument with a required maximum version
  (instead of a supplied version number).

  Also remove unused function STEPMAKE_CHECK_VERSION_UNSUPPORTED.