[zeromq-dev] Do mention `xmlto' in configure warning

[Vitaly Mayatskikh]

./configure checks for availability of both asciidoc and xmlto, but
complains only for missing asciidoc, even if asciidoc is present and
xmlto is missing.

Submitting under MIT.

diff --git a/configure.in b/configure.in
index 36f95a8..eb36f8f 100644
--- a/configure.in
+++ b/configure.in
@@ -84,7 +84,7 @@ if test -d ".git"; then
     if test "x$have_asciidoc" = "xno" -o "x$have_xmlto" = "xno"; then
         build_doc="no"
         install_man="no"
-        AC_MSG_WARN([You are building from git and asciidoc is not installed. Documentation will not be built or installed.])
+        AC_MSG_WARN([You are building from git and asciidoc or xmlto is not installed. Documentation will not be built or installed.])
     else
         build_doc="yes"
     fi

-- 
wbr, Vitaly