Fix warning message of autogen.sh for python-binding.

[Geert Janssens]

On Monday 24 May 2010, Tao Wang wrote:
> Hi,
> 
> When run autogen.sh, it complains:
> 
> src/optional/python-bindings/Makefile.am:3: variable `SWIG_SOURCES' is
> defined but no program or
> src/optional/python-bindings/Makefile.am:3: library has `SWIG' as
> canonical name (possible typo)
> 
> Should not use SWIG_SOURCES for *.i which conflict with automake's
> naming schema. I created a patch to change it to SWIG_FILES.
> 
Thank you for your patch.

I applied it as r19203.

Geert