Create aliases for probed variables

Headers from other libraries (notably DUNE) require definitions of
other variables than our probe process set (e.g. HAVE_UMFPACK instead
of HAVE_SUITESPARSE_UMFPACK_H). This module sets those aliases for
compatibility.
Declare a need to know if UMFPACK is available

Note that the variable HAVE_UMFPACK is not set by FindSuiteSparse,
but by OpmAliases.