Bump version requirements to base>=4.6, Cabal>=1.20

Cabal>=1.20 allows control over parallelism when compiling grammars from
Setup.hs and WebSetup.hs.

base>=4.6 allows conditional compilation with CPP to be eliminated from
a few modules.

base-4.6 corresponds to GHC 7.6.3, which is what you get in
Debian 8 (aka jessie, aka oldstable) from 2015.

src/compiler/GF/System/Catch.hs

@@ -1,13 +1,7 @@
--- | Isolate backwards incompatible library changes to 'catch' and 'try'
-{-# LANGUAGE CPP #-}
+-- | Backwards compatible 'catch' and 'try'
 module GF.System.Catch where
 import qualified System.IO.Error as S
 
 -- ** Backwards compatible try and catch
-#if MIN_VERSION_base(4,4,0)
 catch = S.catchIOError
 try = S.tryIOError
-#else
-catch = S.catch
-try = S.try
-#endif

src/compiler/GF/System/Concurrency.hs

@@ -1,7 +1,6 @@
-{-# LANGUAGE CPP,ForeignFunctionInterface #-}
+{-# LANGUAGE ForeignFunctionInterface #-}
 -- | A variant of 'Control.Concurrent.setNumCapabilities' that automatically
--- detects the number of processors in the system, and is available
--- even when compiling with GHC<7.6.
+-- detects the number of processors in the system.
 module GF.System.Concurrency(
   -- * Controlling parallelism
   setNumCapabilities,getNumberOfProcessors) where
@@ -16,13 +15,8 @@ import Foreign.C.Types(CInt(..))
 -- hasn't already been set with @+RTS -N/n/ -RTS@.
 setNumCapabilities opt_n =
   do n <- maybe getNumberOfProcessors return opt_n
-#if MIN_VERSION_base(4,6,0)
      C.setNumCapabilities n
      return True
-#else
-     n_now <- C.getNumCapabilities
-     return (n==n_now)
-#endif
 
 -- | Returns the number of processors in the system.
 getNumberOfProcessors = fmap fromEnum c_getNumberOfProcessors

src/runtime/haskell/PGF/Optimize.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE CPP, BangPatterns #-}
+{-# LANGUAGE BangPatterns #-}
 module PGF.Optimize
              ( optimizePGF
              , updateProductionIndices
@@ -11,11 +11,7 @@ import PGF.Macros
 import Data.List (mapAccumL)
 import Data.Array.IArray
 import Data.Array.MArray
-#if MIN_VERSION_base(4,6,0)
 import Data.Array.Unsafe as U(unsafeFreeze)
-#else
-import Data.Array.ST as U(unsafeFreeze)
-#endif
 import Data.Array.ST
 import Data.Array.Unboxed
 import qualified Data.Map as Map