Backport suitesparse test to 2.4
This is a straightforward backport of the new SuiteSparse test from master by simply cherry-picking all commits that touched either
FindUMFPack.cmake. On top of that, I removed the deprecation warning from the old UMFPack test and extended it to automatically populate
UMFPACK_ROOT if only the latter is set.
This fixes #14 (closed)