[GiNaC-list] cmake patch to find correct python executable

Hi all, Please consider including the attached patch. With this patch, cmake searches for the correct path of the python2 interpreter and uses that path for calling python, instead of just calling "python". This fixes a bug on Fedora, which no longer has an unversioned python interpreter (i.e., only python2 and python3). It should also avoid using python3 on accident. On a related note, are there any plans to switch to python3? I only had a quick glance and there are some incompatibilities, but most of them seem to be rather straight-forward to fix. Thanks! Till