configure --enable-python doesn't check for availability of swig

To build wiredtiger with python support swig is one of the dependencies.
configure script at the moment doesn't check for the swig's availability.

Build eventually fails with the following error if swig is unavailable:

make[2]: Entering directory '/home/sulabh/work/wt2319/build_posix/lang/python'
(cd ../../../lang/python && \
-python -threads -O -Wall -nodefaultctor -nodefaultdtor -I/home/sulabh/work/wt2319/build_posix wiredtiger.i && \
mv wiredtiger.py wiredtiger/_init_.py)
/bin/bash: line 1: -python: command not found

This ticket tracks improvements required to the configure script.
Script should check for swig's availability in case --enable-python flag is passed, and fail if not found.