.dir-locals.el: make it apply to c++ instead of c

On Kubuntu 14.04 this suddenly caused problems...
build system: Specifying SOURCES for tests which are not compiled is superfluous
partial relinearization: use better for the message at the end of a Newton iteration
decouple thermodynamic hints from the volume variables cache

i.e., it is now possible to enable the former but disable the
latter. Both mechanisms still use the same storage space, though...
Export HAVE_DUNE_ISTL for other modules.

This is e.g. needed in the parallel version of autodiff to
test whether ISTL is there.
syncronize the support for UG with opm-core
Search for MPI support

The build system depends on knowing if MPI is available.  Explicitly
search for MPI to honour that requirement.  Don't rely on ISTL's
transitive searching for the same.
do not add the "-Wl,--as-needed" linker flag in FindSuiteSparse.cmake

because if umfpack is fully linked but some other library is
underlinked, it causes the build to fail. This flag is better dealt
with in the "UseOnlyNeeded.cmake" module...
make the "-Wl,--as-needed" flag optional

as there is not much harm done if it is not used but it may cause the
build to fail if it is present (read: it causes _my_ build to fail),
it defaults to OFF.
syncronize the build system with opm-core