change CPP to something more semantically reasonable

I'm not sure how the previous version worked, to be honest.

src/Graphics/Svg/Core.hs

@@ -40,7 +40,7 @@ import           Data.ByteString.Lazy (ByteString)
 import           Data.Hashable (Hashable(..))
 import           Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as M
-#if __GLASGOW_HASKELL__ != 710
+#if !MIN_VERSION_base(4,8,0)
 import           Data.Monoid (Monoid(..))
 #endif
 import           Data.Semigroup (Semigroup(..))