Inject SQL_CALC_FOUND_ROWS into db_select()?
As made evident in Compare db_query() and db_select() performance, db_query() is faster than db_select(), which should then be used when you need to make the query you are executing alterable from other module via hook_query_alter(), for example when you need to get a list of nodes for which a user has access.

If that is not your case, then you can use db_query() and using the query you are showing is quite straight as db_query("SELECT SQL_CALC_FOUND_ROWS field1, field2 FROM ...").