non-obvious ./configure error message when asciidoc isn't installed
I just saw a user build Tor from git, and he didn't have asciidoc installed, and ./configure gave him
echo "==================================";
echo;
echo "You need asciidoc installed to be able to build the manpage.";
echo "To build without manpages, use the --disable-asciidoc argument";
echo "when calling configure.";
echo;
echo "==================================";
exit 1;
which he interpreted as a successful run. We should make the above message more clear that it's a failure case.