On AIX I want to compile with multi-threading, then means
using xlC_r.

Using
      bjam threading=multi

Will do this.

However is there a way achieving the same effect without
putting "threading=multi" on the command line ?
i.e putting the threading model in site-config.jam

In my site-config.jam , have tried the following, all to no avail

using xlc : : xlC_r ;
using vacpp  : : <threading>multi ;
using vacpp : xlC_r ;

  Best regards,
Ta,
   Avi