Fix for pre-8.0 GHCs.

This adds a semigroups dependency, which might be annoyingly heavy;
the alternative is more CPP.

doctest.cabal

@@ -2,7 +2,7 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 65d4f9a2ccf1a31ae95a19e970e90bad28f12dcbbfca47bb02089ea58d78b696
+-- hash: 7ba3485a48f328a133e6770af202d9940cefa72c268fddf5af01b37f38c4d42c
 
 name:           doctest
 version:        0.14.0
@@ -66,6 +66,7 @@ library
     , ghc >=7.0 && <8.6
     , ghc-paths >=0.1.0.9
     , process
+    , semigroups
     , syb >=0.3
     , transformers
   default-language: Haskell2010
@@ -88,6 +89,7 @@ executable doctest
     , ghc >=7.0 && <8.6
     , ghc-paths >=0.1.0.9
     , process
+    , semigroups
     , syb >=0.3
     , transformers
   default-language: Haskell2010
@@ -109,6 +111,7 @@ test-suite doctests
     , ghc >=7.0 && <8.6
     , ghc-paths >=0.1.0.9
     , process
+    , semigroups
     , syb >=0.3
     , transformers
   default-language: Haskell2010
@@ -169,6 +172,7 @@ test-suite spec
     , hspec >=1.5.1
     , mockery
     , process
+    , semigroups
     , setenv
     , silently >=1.2.4
     , stringbuilder >=0.4

package.yaml

@@ -35,6 +35,7 @@ dependencies:
 - filepath
 - process
 - ghc-paths >= 0.1.0.9
+- semigroups
 - transformers
 
 library:

src/Runner.hs

@@ -14,10 +14,11 @@ module Runner (
 import           Prelude hiding (putStr, putStrLn, error)
 
 #if __GLASGOW_HASKELL__ < 710
-import           Data.Monoid
+import           Data.Monoid hiding ((<>))
 import           Control.Applicative
-#else
-import Data.Semigroup
+#endif
+#if !MIN_VERSION_base(4,11,0)
+import           Data.Semigroup
 #endif
 
 import           Control.Monad hiding (forM_)