make web doesn't handle modified files

[Graham Percival]

"make web" doesn't notice that source files have been modified since 
the last build.  I just tried inserting some bad input
@example
{
c a b
}
@end example
(instead of @{ @})

into tutorial.itely, and make web happily chugged away and didn't 
display any error messages.  I've also found that good input doesn't 
end up in new html files.

- Graham