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.

src/compiler/GF/Infra/BuildInfo.hs

@@ -18,6 +18,9 @@ buildInfo =
 #endif
 #ifdef NEW_COMP
     ++" new-comp"
+#endif
+#ifdef C_RUNTIME
+    ++" c-runtime"
 #endif
   where
     details = either (const no_info) info darcs_info