Removed custom Setup.hs, use build-type: simple.

The only thing we gained from the custom build was
automatic installation of the man page when using
'cabal install'.  But custom builds cause problems,
e.g., with cross-compilation.

Installation of the man page is better handled by packagers.

Note to packagers (e.g. Debian):  it may be necessary
to add a step installing the man page with the next
release.

Setup.hs

@@ -1,38 +1,2 @@
-{-
-Copyright (C) 2006-2018 John MacFarlane <[email protected]>
-
-This program is free software; you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation; either version 2 of the License, or
-(at your option) any later version.
-
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-GNU General Public License for more details.
-
-You should have received a copy of the GNU General Public License
-along with this program; if not, write to the Free Software
-Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
--}
-
 import Distribution.Simple
-import Distribution.Simple.Setup (CopyFlags(..), fromFlag)
-import Distribution.PackageDescription (PackageDescription(..))
-import Distribution.Simple.Utils (notice, installOrdinaryFiles)
-import Distribution.Simple.LocalBuildInfo
-
-main :: IO ()
-main = defaultMainWithHooks $ simpleUserHooks {
-      postCopy = installManPage
-    }
-
-installManPage :: Args -> CopyFlags
-               -> PackageDescription -> LocalBuildInfo -> IO ()
-installManPage _ flags pkg lbi = do
-  let verbosity = fromFlag (copyVerbosity flags)
-  let copydest  = fromFlag (copyDest flags)
-  let mandest   = mandir (absoluteInstallDirs pkg lbi copydest)
-                     ++ "/man1"
-  notice verbosity $ "Copying man page to " ++ mandest
-  installOrdinaryFiles verbosity mandest [("man", "pandoc.1")]
+main = defaultMain

pandoc.cabal

@@ -1,7 +1,7 @@
 name:            pandoc
 version:         2.6
 cabal-version:   2.0
-build-type:      Custom
+build-type:      Simple
 license:         GPL-2
 license-file:    COPYING.md
 copyright:       (c) 2006-2018 John MacFarlane
@@ -346,9 +346,6 @@ flag trypandoc
   Description:   Build trypandoc cgi executable.
   Default:       False
 
-custom-setup
-  setup-depends: base, Cabal >= 2.0
-
 library
   build-depends: base >= 4.8 && < 5,
                  syb >= 0.1 && < 0.8,