gf -cshell: preliminary support for the C run-time system in the GF shell

Some C run-time functionality is now available in the GF shell, by starting
GF with 'gf -cshell' or 'gf -crun'. Only limited functionality is available
when running the shell in these modes:

- You can only import .pgf files, not source files.
- The -retain flag can not be used and the commands that require it to work
  are not available.
- Only 18 of the 40 commands available in the usual shell have been
  implemented. The 'linearize' and 'parse' commands are the only ones
  that call the C run-time system, and they support only a limited set of
  options and flags. Use the 'help' commmands for details.
- A new command 'generate_all', that calls PGF2.generateAll, has been added.
  Unfortuntaly, using it causes 'segmentation fault'.

This is implemented by adding two new modules: GF.Command.Commands2 and
GF.Interactive2. They are copied and modified versions of GF.Command.Commands
and GF.Interactive, respectively. Code for unimplemented commands and other
code that has not been adapted to the C run-time system has been left in
place, but commented out, pending further work.

src/compiler/GF/Main.hs

@@ -1,7 +1,11 @@
 -- | GF main program (grammar compiler, interactive shell, http server)
+{-# LANGUAGE CPP #-}
 module GF.Main where
 import GF.Compiler
-import GF.Interactive
+import qualified GF.Interactive as GFI1
+#ifdef C_RUNTIME
+import qualified GF.Interactive2 as GFI2
+#endif
 import GF.Data.ErrM
 import GF.Infra.Option
 import GF.Infra.UseIO
@@ -43,7 +47,17 @@ mainOpts opts files =
     case flag optMode opts of
       ModeVersion     -> putStrLn $ "Grammatical Framework (GF) version " ++ showVersion version ++ "\n" ++ buildInfo
       ModeHelp        -> putStrLn helpMessage
-      ModeInteractive -> mainGFI opts files
-      ModeRun         -> mainRunGFI opts files
-      ModeServer port -> mainServerGFI opts port files
+      ModeServer port -> GFI1.mainServerGFI opts port files
       ModeCompiler    -> mainGFC opts files
+      ModeInteractive -> GFI1.mainGFI opts files
+      ModeRun         -> GFI1.mainRunGFI opts files
+#ifdef C_RUNTIME
+      ModeInteractive2 -> GFI2.mainGFI opts files
+      ModeRun2         -> GFI2.mainRunGFI opts files
+#else
+      ModeInteractive2 -> noCruntime
+      ModeRun2         -> noCruntime
+  where
+    noCruntime = do ePutStrLn "GF configured without C run-time support"
+                    exitFailure
+#endif