* scripts/lilypond-book.py (g_make_html): New global variable.

* scripts/lilypond-book.py (g_make_html): New global variable.
(schedule_lilypond_block): Don't compute EPS and PNG for `texi'
if `g_make_html' isn't set.
[main]: Add format `texi-html'.
(option_definitions): Updated.
* Documentation/user/lilypond-book.itely: Updated.

* make/lilypond-vars.make (LILYPOND_BOOK_FORMAT): New variable.
* make/ly-rules.make (%.texi, %.nexi): Use it.
* stepmake/stepmake/www-targets.make (web): Pass `tex-html' as
format to lilypond-book.
* GNUmakefile.in (web-doc): Ditto.