Exclude examples/sim_simple.cpp if UMFPACK is unavailable

Not all systems provide UMFPACK so even if Eigen provides the required
bindings, we should not assume that the underlying library and/or
headers are available.  Currently, only examples/sim_simple.cpp uses
UMFPACK unconditionally, so the simplest solution is to exclude that
example unless UMFPACK is available.
Merge branch 'master' into optional-umfpack

Conflicts:
	CMakeLists.txt

Import akva2's modified Eigen support.
Fix mismerge in commit b54df70.

We lost the SuiteSparse detection.
Merge pull request #15 from bska/optional-umfpack

Exclude examples/sim_simple.cpp if UMFPACK is unavailable