Emit a warning when -g was not passed (#268)

Fixes #268.

src/Main.lhs

@@ -25,7 +25,7 @@ Path settings auto-generated by Cabal:
 > import Happy.Tabular.Info (genInfoFile)
 
 > import System.Console.GetOpt
-> import Control.Monad ( liftM, when )
+> import Control.Monad ( liftM, when, unless )
 > import System.Environment
 > import System.Exit (exitWith, ExitCode(..))
 > import Data.Char
@@ -62,6 +62,11 @@ Read and parse the CLI arguments.
 >  where
 >    runParserGen cli fl_name = do
 
+If no -g flag has been passed, show a warning.
+
+>       unless (OptGhcTarget `elem` cli) $
+>           hPutStrLn stderr "Warning: With happy 2.0, the --ghc flag has become non-optional. To suppress this warning, pass the --ghc flag."
+
 Open the file.
 
 >       fl <- readFile fl_name