gf.cabal: removing -O2

Removing "ghc-options: -O2" from gf.cabal has the following advantages:

+ Compiling GF is faster (time drops from 182s to 142s on my laptop)
+ Compiling the RGL is faster (time drops from 159s to 155s on my laptop)
+ Without the hardwired optimization level, the 'cabal configure' options
  --enable-optimization=<n> and --disable-optimization work as expected
  (so if you still want -O2, use --enable-optimization=2)
+ GF can be compiled with ghc-7.2.2 and ghc-7.4.1 (-O2 triggers a bug in
  these versions of ghc, it seems. Another workaround, discovered by
  Sergei Trofimovich, is to use -O0 in Data.Binary.)

gf.cabal

@@ -50,7 +50,7 @@ library
                  random,
                  pretty,
                  mtl
-  ghc-options: -O2
+--ghc-options: -O2
   hs-source-dirs: src/compiler src/runtime/haskell
   extensions: 
   exposed-modules: 
@@ -119,7 +119,7 @@ executable gf
 
   ghc-prof-options: -auto-all
 
-  ghc-options: -O2
+--ghc-options: -O2
   if impl(ghc>=7.0)
     ghc-options: -rtsopts
   hs-source-dirs: src/compiler src/runtime/haskell