futhark: introduce the top-level futhark attribute

The fact that futhark is a Haskell package is an implementation detail. To
install it users should just have to specify `futhark` instead of
`haskellPackages.futhark`.

Additionally futhark is overridden with `haskell.lib.justStaticExecutables` to
reduce closure size.

pkgs/development/haskell-modules/configuration-common.nix

@@ -707,6 +707,7 @@ self: super: {
   # vaultenv is not available from Hackage.
   vaultenv = self.callPackage ../tools/haskell/vaultenv { };
 
+  # futhark is not available from Hackage.
   futhark = self.callPackage ../compilers/futhark { };
 
   # https://github.com/Philonous/hs-stun/pull/1

pkgs/development/haskell-modules/configuration-nix.nix

@@ -528,4 +528,19 @@ self: super: builtins.intersectAttrs super {
 
   # The test-suite requires a running PostgreSQL server.
   Frames-beam = dontCheck super.Frames-beam;
+
+  futhark = with pkgs;
+    let path = stdenv.lib.makeBinPath [ gcc ];
+    in overrideCabal (addBuildTool super.futhark makeWrapper) (_drv: {
+      postInstall = ''
+        wrapProgram $out/bin/futhark-c \
+          --prefix PATH : "${path}"
+
+        wrapProgram $out/bin/futhark-opencl \
+          --prefix PATH : "${path}" \
+          --set NIX_CC_WRAPPER_x86_64_unknown_linux_gnu_TARGET_HOST 1 \
+          --set NIX_CFLAGS_COMPILE "-I${opencl-headers}/include" \
+          --set NIX_CFLAGS_LINK "-L${opencl-icd}/lib"
+      '';
+    });
 }