SWIG versions - summary of previous thread, and moving forward

The overall status coming out of the previous thread, IIUC, is:

 * There's no particular known reason not to increase the maximum
   acceptable version to SWIG 1.3.31 right now.

     - Accordingly, I'll make that change.

     - And furthermore, I suggest we update the release manager
       designated version of SWIG from 1.3.25 to 1.3.31 for Subversion
       1.5.x.

 * We cannot immediately raise our minimum requirement, because the
   Windows builds are known to have problems with anything other than
   1.3.24. NB: the Windows distribtion does _not_ ship pregenerated SWIG
   files in the zipfile, unlike the Unix distribution.

     - Hopefully at some point someone will resolve whatever problem is
       keeping Windows builds from using SWIG >> 1.3.24, and at that
       point we can raise our minimum SWIG version to 1.3.29 initially.

Max.