docs: prefer to use rst2html5 instead of rst2html

Our website is written assuming HTML5 standard & doctype:

  commit b1c81567
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Jul 26 18:01:25 2017 +0100

    docs: switch to using HTML5 doctype declaration

so we want the RST conversion to also use HTML5. Ubuntu 16.04 still
only has the HTML4 generating tools though, so we have that as a
fallback.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

m4/virt-external-programs.m4

@@ -33,10 +33,13 @@ AC_DEFUN([LIBVIRT_CHECK_EXTERNAL_PROGRAMS], [
   then
     AC_MSG_ERROR("xsltproc is required to build libvirt")
   fi
-  AC_PATH_PROGS([RST2HTML], [rst2html rst2html.py rst2html-3], [])
+
+  dnl Drop the rst2html (aka HTML4) variants once we
+  dnl stop supporting Ubuntu 16.04 (Xenial)
+  AC_PATH_PROGS([RST2HTML], [rst2html5 rst2html5.py rst2html5-3 rst2html rst2html.py rst2html-3], [])
   if test -z "$RST2HTML"
   then
-    AC_MSG_ERROR("rst2html is required to build libvirt")
+    AC_MSG_ERROR("rst2html5/rst2html is required to build libvirt")
   fi
   AC_PATH_PROG([AUGPARSE], [augparse], [/usr/bin/augparse])
   AC_PROG_MKDIR_P