Error when no rst2man is found

If one runs configure with --without-rst2man or no rst2man is found,
the value is "no", not an empty string.  Handle this correctly in
configure.

Fixes: #1473

configure.ac

@@ -55,7 +55,7 @@ AC_ARG_WITH([rst2man],
   AS_HELP_STRING([--with-rst2man=PATH], [Location of rst2man (auto)]),
   [RST2MAN="$withval"],
   AC_CHECK_PROGS(RST2MAN, [rst2man rst2man.py], []))
-if test -z "$RST2MAN"; then
+if test "$RST2MAN" = "no"; then
   AC_MSG_ERROR(
     [rst2man is needed to build Varnish, please install python-docutils.])
 fi