Making texi2html optional?

[John Mandereau]

Hi,

I'm afraid we might wait a release of texi2html for a while (several
weeks).  We can't require texi2html in Lily 2.12 if it hasn't been
officially released; OTOH Reinhold has put a lot of effort into using
texi2html for building our docs, so we really should release 2.12. HTML
documentation compiled with texi2html, and thus one or two 2.11.x
releases for testing.  That's why I suggest to make texi2html optional
in configure script, and write conditional make rules for building HTML
docs -- the rules would be defined conditionnally like

ifeq (,$(findstring texi2html,$(MISSING_OPTIONAL)))
# rules using texi2html
else
# rules using makeinfo
endif

Then we could immediately merge dev/texi2html into master.  I can
provide a patch prior to pushing if you like, but I think it's not too
much of a burden: only texinfo-rules.make and doc-i18n-user-rules.make
would be affected besides configure.in.

Cheers,
John