Makes KAMG and FastAMG solver available with dune-istl 2.3

Previously we relied on the define DUNE_HAS_FAST_AMG to detect
whether these preconditioners are available. This define is only
available in the 2.2 release with cmake support. Therfore we now
addtionally test whether we are using dune-isl 2.3 or newer.
[cmake] do not export dune-istl version but use module local detection.

According to @rolk defines should only be exported in
cmake/Modules/<module>-prereqs.cmake if they are used in the
headers. This is not the case for the dune-istl version macros and
therefore we use opm_need_version_of in the config_hook in the top
level CMakefile with this patch.
Merge pull request #530 from blattms/support-fast-amg-2.3

Makes KAMG and FastAMG solver available with dune-istl 2.3