remove template haskell dependency in favor of cabal's CPP macro

It's possible to have gotten the wrong ghc version with the old method,
if somebody does `cabal install IHaskell --with-ghc=/path/to/another/ghc`

IHaskell.cabal

@@ -73,8 +73,7 @@ library
                        system-filepath,
                        cereal ==0.3.*,
                        text >=0.11,
-                       mtl >= 2.1,
-                       template-haskell
+                       mtl >= 2.1
   exposed-modules: IHaskell.Display,
                    Paths_ihaskell,
                    IHaskell.Types,
@@ -132,8 +131,7 @@ executable IHaskell
                        system-filepath,
                        cereal ==0.3.*,
                        text >=0.11,
-                       mtl >= 2.1,
-                       template-haskell
+                       mtl >= 2.1
 
 Test-Suite hspec
   Type:     exitcode-stdio-1.0
@@ -166,8 +164,7 @@ Test-Suite hspec
                        system-filepath,
                        cereal ==0.3.*,
                        text >=0.11,
-                       mtl >= 2.1,
-                       template-haskell
+                       mtl >= 2.1
 
 source-repository head
   type:     git

IHaskell/Message/Writer.hs

-{-# LANGUAGE NoImplicitPrelude, OverloadedStrings #-}
-{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE CPP, NoImplicitPrelude, OverloadedStrings #-}
 -- | Description : @ToJSON@ for Messages
 --
 -- This module contains the @ToJSON@ instance for @Message@.
@@ -11,20 +10,16 @@ import Prelude (read)
 import ClassyPrelude
 import Data.Aeson
 
-import Language.Haskell.TH
 import Shelly hiding (trace)
 
 import IHaskell.Types
 
--- | Compute the GHC API version number using Template Haskell.
+-- | Compute the GHC API version number using the dist/build/autogen/cabal_macros.h
 ghcVersionInts :: [Int]
-ghcVersionInts = ints . map read . words . map dotToSpace $ version
+ghcVersionInts = ints . map read . words . map dotToSpace $ VERSION_ghc
   where dotToSpace '.' = ' '
         dotToSpace x = x
 
-        version :: String
-        version = $(runIO (unpack <$> shelly (run "ghc" ["--numeric-version"])) >>= stringE)
-
 -- Convert message bodies into JSON.
 instance ToJSON Message where
   toJSON KernelInfoReply{} = object [