Add path to installed libraries to ffi.py

[dsanderson]

By default, libfive/bin/python/ffi.py searches for the libfive and libfive-stdlib libraries in the relative paths . and libfive; as a consequence, from libfive import stdlib fails if the script is not run in the build directory.

Cmake installs the libraries to ${CMAKE_INSTALL_PREFIX}/lib. I added that path to the paths variable in ffi.py, using the cmake configure_file function to replace the ${CMAKE_INSTALL_PREFIX} variable with the correct path. As a consequence, ffi.py is now copied into the build directory by cmake, rather than deferring to make. I don't know if that approach is acceptable here. Additionally, I've only been able to test this on linux (Ubuntu 20.04), so I put the changes to ffi.py behind a system check.

This change means, after running make install, the python bindings can be used from anywhere on the system.