Optionally include C run-time support

If the C run-time library is compiled and installed on your system, you can now
do 'cabal configure -fc-runtime' to get the following extras:
  + The haskell binding to the C run-time library will be included in the
    PGF library (so you can import it in Haskell applications).
    Documentation on the new modules will be included when you run
    'cabal haddock'.
  + The new command 'pgf-shell', implemented on top of haskell binding to
    the C run-time system.
  + Three new commands in the web API: c-parse, c-linearize and
    c-translate. Their interfaces are similar to the corresponding commands
    without the "c-" prefix, but they should be considered preliminary.

Setup.hs

@@ -351,8 +351,12 @@ run_gfc pkg lbi args =
 
 default_gf pkg lbi = buildDir lbi </> exeName' </> exeNameReal
   where
+    exeName' = "gf"
+    exeNameReal = exeName' <.> exeExtension
+    {- --old solution, could pick the wrong executable if there is more than one
     exeName' = (exeName . head . executables) pkg
     exeNameReal = exeName' <.> (if null $ takeExtension exeName' then exeExtension else "")
+    -}
 
 -- | Create autogen module with detailed version info by querying darcs
 extractDarcsVersion distFlag =