- replace usage of %%define with %%global
    - use the %%{_isa} macro to ensure that the python-devel dependency on
        python is for the correct multilib arch (#555943)
    - delete bundled copy of libffi to make sure we use the system one
    - replace references to /usr with %%{_prefix}; replace references to
        /usr/include with %%{_includedir}