Enables FastAMG for dune-istl version >=2.3, too.

The HAS_DUNE_FAST_AMG is only available in the 2.2 cmake branches.
This patch checks whether dune-istl is of version 2.3 or higher and
in this case activates FastAMG, too.
Added missing include for version check
Merge pull request #90 from blattms/fixes_fastamg_with_dune-istl2.3

Enables FastAMG for dune-istl version >=2.3, too.