Previously, we said (more or less), "a2x is broken and here's how you could
try to fix it". Instead, we now say "We need a2x to build manpages; a2x
didn't work; here is a fix that might work for you; alternatively you
could just skip manpage building."
Addresses bug 1524.
Also, give the message as a here-document rather than a bunch of echos.
Zax from #tor noticed that while we list docbook-xsl and docbook-xml
in our helpful error text when making the docs fails, we forgot to
also list libxml2-utils. Let's add that.
