CMakeLists: Find Python 2.* libs

When Python 2 and 3 are installed alongside on a system, one may get the following configuration:

================ *snip* ================
[...]
-- Found PythonInterp: /usr/bin/python (found version "2.7.3")
-- Found PythonLibs: /usr/lib/python3.2/config/libpython3.2.so (found version "2.7.3")
[...]
================ *snap* ================

If I recall correctly, FEniCS works only with Python 2, so you might want to add the explicit version specification in the find_package call for the Python libs, i.e.,

  find_package(PythonLibs 2)

--Nico