RSB options for python selection

A recent change to the version of GDB means we need python3 development libraries installed as GDB will only build with Python3.

To aid compatibility for older operating systems provide options to aid the selection of a suitable python. Add support for:

--with-python2 : command for Python2

--with-python3 : command for Python3

--with-python-version : command for a version of Python

For example:

--with-python-version=python3.6