server-variables in RewriteCond?

I've been trying to do this:

RewriteCond %{HTTP_HOST} !%{SERVER_NAME}
RewriteRule ^(.*)$ http://%{SERVER_NAME}$1 [R=301,L]

as a generic 301 all host names to the definitive server name, but it
appears that I cannot simply insert a server-variable in the RHS of the
RewriteCond.

Is there any way round this, apart from using the actual server name on the
RHS?