added: hook for conditionally compiled files

backport from opm-upscaling PR#100
Merge pull request #602 from akva2/add_files_hook

added: hook for conditionally compiled files
fixed: dune-cornerpoint (now) needs the dune-common version information
Merge pull request #603 from akva2/fix_cp_prereqs

fixed: dune-cornerpoint (now) needs the dune-common version information
Findopm-parser.cmake: Fix it up, next try

thanks to Arne Morten Kvarving, Bård Skaflestad and Roland Kaufmann
for the discussion.