Building doc: manual.txt success despite missing manual.html

[Moritz Barsnick <barsnick@xxxxxxx>]

Hi,

I just noticed a peculiar one:
When building the documentation, neither lynx nor w3m bomb out on
missing input files. So both
lynx -dump -nolist -with_backspaces manual.html > manual.txt
and
w3m -dump manual.html > manual.txt
have a return value of 0. Thus make succeeds, and I have an empty
manual. :-) The "manual.txt" make rule properly has a dependency on
"manual.html", but that doesn't seem to help. ;-)

BTW, the root cause of this is that my xsltproc is failing - due to
somewhat obvious reasons properly described in doc/devel-notes.txt.

Moritz :)