Documentation: Modify rule for html output (better looking output format)

Documentation: Modify rule for html output (better looking output format)

To create html output from texi input, texi2html was used.
Output from makeinfo looks cleaner, so replace the old rule
and use makeinfo now.

For those who want to use their own variant of html output,
the macros MAKEINFO and MAKEINFOFLAGS allow customisation.
Option "-I ." is not needed (the current directory is
searched by default), so remove it.

Please note that the build requirements changed, too:
makeinfo is required for doc builds.
texi2html is no longer used.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>