Switch to custom Setup.hs file for cabal-doctests

doctest doesn't respect the `default-extensions` field from the cabal
file, so we previously had to put every extension at the start of each
and every file so that doctests would compile.

Using the `cabal-doctest` package, we can pass the right options to
`doctest` at build time and move extension declarations to the cabal
file.

Setup.hs

+{-# LANGUAGE CPP #-}
+
+module Main (main) where
+
+#ifndef MIN_VERSION_cabal_doctest
+#define MIN_VERSION_cabal_doctest(x,y,z) 0
+#endif
+
+#if MIN_VERSION_cabal_doctest(1,0,0)
+
+import Distribution.Extra.Doctest ( defaultMainWithDoctests )
+main :: IO ()
+main = defaultMainWithDoctests "doctests"
+
+#else
+
+#ifdef MIN_VERSION_Cabal
+-- If the macro is defined, we have new cabal-install,
+-- but for some reason we don't have cabal-doctest in package-db
+--
+-- Probably we are running cabal sdist, when otherwise using new-build
+-- workflow
+#warning You are configuring this package without cabal-doctest installed. \
+         The doctests test-suite will not work as a result. \
+         To fix this, install cabal-doctest before configuring.
+#endif
+
 import Distribution.Simple
+
+main :: IO ()
 main = defaultMain
+
+#endif

ma.cabal

@@ -11,8 +11,12 @@ author:              Hans-Peter Deifel
 maintainer:          hans-peter.deifel@fau.de
 -- copyright:
 -- category:
-build-type:          Simple
-cabal-version:       >=1.10
+build-type:          Custom
+cabal-version:       >=1.24
+                     
+custom-setup
+  setup-depends:
+    base, Cabal, cabal-doctest >= 1.0.2 && <1.1
 
 library
   hs-source-dirs:      src

tests/DocTests.hs

+{-# LANGUAGE GADTs #-}
 module Main where
 
-import Test.DocTest
+import Build_doctests (flags, pkgs, module_sources)
+import Data.Foldable (traverse_)
+import Test.DocTest (doctest)
 
-main = doctest ["src/"]
+main :: IO ()
+main = doctest (flags ++ pkgs ++ module_sources)