configure: fix python major version check on python 2.6

Python 2.6 doesn't use a named tuple for the version info,
instead use the index of the major version which works
on Python 2.6 upwards.

configure.ac

@@ -124,7 +124,7 @@
     # Get the Python major version. This is only for information
     # messages displayed during configure.
     if test "x$HAVE_PYTHON" != "xno"; then
-       pymv="$($HAVE_PYTHON -c 'import sys; print(sys.version_info.major);')"
+       pymv="$($HAVE_PYTHON -c 'import sys; print(sys.version_info[[0]]);')"
     fi
 
     # Check for python-distutils (setup).