Complete cf7f07b PR #530 to support FastAMG with dune-istl 2.3

This fixes further occurences of DUNE_HAS_FASTAMG that were
missed in pull request #530.

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-istl 2.3 or newer.
Include fastamg.hh for dune-istl 2.3

For the inofficial 2.2 release this was included automatically with amg.hh.
Merge pull request #531 from blattms/support-fast-amg-2.3-part2

Complete cf7f07b PR #530 to support FastAMG with dune-istl 2.3