SuiteSparse may or may not be installed in a suitesparse/ directory.
FindSuiteSparse will look in a suitesparse/ subdirectory when trying
to locate umfpack.h, but it will add the full directory to the compiler
command-line (e.g. `-I/usr/include/suitesparse`) and not that of the
parent. Since the parent is usually included too, it is not noticed
that it is advertedly using another include paths than its own.
However, if we have SuiteSparse installed in a non-system location,
using the subdirectory in the `#include` statement may now cause an
error, even though configuration actually found SuiteSparse!
