cabal init: simple heuristic for guessing common extra-source-files
Drop code to handle unsupported compilers

This means tweaking CPP conditionals and file pragmas to remove handling
of compilers other than GHC, and GHC versions before LANGUAGE pragmas
existed.
Merge pull request #1166 from benmachine/nohugs

Drop code to handle unsupported compilers
Merge pull request #1157 from byorgey/master

Add simple heuristic to cabal init to guess common extra-source-files