Modifications to load CUDD library on Mac from MacPorts.

Change the loading directions to permit CFFI to find the CUDD library
using its defaulting strategies.  This allows it to find "libcudd.dylib"
on Mac and "libcudd.so" on Linux.

Add code to find libcudd where MacPorts stashes it.  Should be a no-op
if it's not there.