Improve the path-finding for FindSUNDIALS.cmake.
Improve the path-finding for FindSUNDIALS.cmake.

This fixes an issue where if SUNDIALS_DIR is never set then SUN_INC is just
'/include/', which is usually not a directory.

sairajat/dealii,sairajat/dealii,naliboff/dealii,naliboff/dealii,naliboff/dealii,danshapero/dealii,naliboff/dealii,sairajat/dealii,danshapero/dealii,sairajat/dealii,sairajat/dealii,naliboff/dealii,danshapero/dealii,naliboff/dealii,danshapero/dealii,sairajat/dealii,danshapero/dealii,danshapero/dealii,sairajat/dealii,danshapero/dealii,naliboff/dealii