Avoid empty strings when --with-packager(-version) is not specified

Make with_packager and with_packager_version default to "no". This way
--without-packager-version (as shorthand for --with-packager(-version)=no)
works correctly too.

Prior to this patch libvirt outputs a line like this when
--with-packager(-version) was not specified

# ./daemon/libvirtd
14:11:15.018: 31796: info : libvirt version: 0.8.8, package:  ()

Now the unspecified parts are correctly omitted.

Reported by Osier Yang.

configure.ac

@@ -32,11 +32,11 @@ AC_SUBST([LIBVIRT_VERSION_NUMBER])
 AC_ARG_WITH([packager],
             [AS_HELP_STRING([--with-packager],
                             [Extra packager name])],
-            [],[])
+            [],[with_packager=no])
 AC_ARG_WITH([packager-version],
             [AS_HELP_STRING([--with-packager-version],
                             [Extra packager version])],
-            [],[])
+            [],[with_packager_version=no])
 if test "x$with_packager" != "xno"
 then
   AC_DEFINE_UNQUOTED([PACKAGER], ["$with_packager"],