support Python 2.7

Module initialization was done differently in Python 2.x.

The code should also work with different Python versions. What ties it
to 2.7 at the moment is only the dlopen(RTLD_GLOBAL) hack that is also
tying it to 3.3 instead of some arbitrary 3.x version.

The latest available Python developer files are picked by configure.
This can be overriden by passing "-D python_version=2.7/3.3".

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>