automake: don't treat warnings as errors

the only effect is that the build will break for people where automake
produces a warning (e.g. due to being newer than the version of the
developer). Developers on the other hand will also fix warnings
without this. (if they intentionally don't, I'll personally spit them
in the eyes and blind them ;)
Use DUNE to detect DUNE stuff

Since the dune M4 macros are not really guaranteed to be stable, let's
just use dune's DUNE_CHECK_ALL m4 macro to do the dirty work of
detecting the dependencies required for DUNE istl.

With this patch the DUNE-istl solvers will be only
available if opm-core is compiled using the dune build
system. (i.e. using 'dunecontrol --module=opm-core all'. This is
reasonable IMO, because only the dune build system knows the canonical
LD and CPP flags required for dune-istl.)

The approach of this patch is to put a m4_ifdef into configure.ac to
detect whether the module is compiled using dunecontrol. Note, that
this only works if opm-core is retrieved via git or else the f*"§$
dunecontrol skips the autogen stage. I'm quite confident that it will
work in the other case if it is manually instructed to run autogen (->
'dunecontrol --only=opm-core autogen'), but I haven't tried...

better solutions than the m4_ifdef are appreciated!