Update SuiteSparse to 4.3.1, update links

Tim Davis moved from Florida to Texas A&M, so some of the SuiteSparse links are now dead, others are redirecting. The first commit here can be safely backported to release-0.3, I believe, but probably better not to backport the second.

There were some undefined symbols on Windows unless I rearranged the shared-library a bit. Would appreciate someone testing on Mac. Passes tests for me on Win32, Win64, and Linux x64.