mueval,diagrams-builder: fix GHC libdir

Rather than hardcoding the path to GHC's libdir which may change, use
`ghc --print-libdir` in wrappers.

Another option would be to expose mkGhcLibdir from generic-builder.nix.

Also add a test for mueval.

pkgs/development/tools/haskell/mueval/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, makeWrapper, haskellPackages, packages ? (pkgs: [])
+{ lib, stdenv, makeWrapper, haskellPackages, packages ? (pkgs: [])
 }:
 
 let defaultPkgs = pkgs: [ pkgs.show
@@ -8,7 +8,6 @@ let defaultPkgs = pkgs: [ pkgs.show
                         ];
     env = haskellPackages.ghcWithPackages
            (pkgs: defaultPkgs pkgs ++ packages pkgs);
-    libDir = "${env}/lib/ghc-${env.version}";
 
 in stdenv.mkDerivation {
   name = "mueval-env";
@@ -17,11 +16,23 @@ in stdenv.mkDerivation {
 
   nativeBuildInputs = [ makeWrapper ];
 
-  buildCommand = ''
+  dontUnpack = true;
+
+  buildPhase = ''
+    runHook preBuild
+
     mkdir -p $out/bin
 
     makeWrapper $mueval/bin/mueval $out/bin/mueval \
-      --set "NIX_GHC_LIBDIR" "${libDir}"
+      --set "NIX_GHC_LIBDIR" "$(${lib.getExe' env "ghc"} --print-libdir)"
+
+    runHook postBuild
+  '';
+
+  doCheck = true;
+
+  checkPhase = ''
+    [[ $($out/bin/mueval -e 42) == 42 ]]
   '';
 
   passthru = { inherit defaultPkgs; };

pkgs/tools/graphics/diagrams-builder/default.nix

@@ -17,13 +17,13 @@ let
   # Used same technique as for the yiCustom package.
   wrappedGhc = ghcWithPackages
     (self: [ diagrams-builder ] ++ extraPackages self);
-  ghcVersion = wrappedGhc.version;
+  ghc = lib.getExe' wrappedGhc "ghc";
 
   exeWrapper = backend : ''
     makeWrapper \
     "${diagrams-builder}/bin/diagrams-builder-${backend}" "$out/bin/diagrams-builder-${backend}" \
-      --set NIX_GHC ${wrappedGhc}/bin/ghc \
-      --set NIX_GHC_LIBDIR ${wrappedGhc}/lib/ghc-${ghcVersion}
+      --set NIX_GHC ${ghc} \
+      --set NIX_GHC_LIBDIR "$(${ghc} --print-libdir)"
   '';
 
   backends = ["svg" "cairo" "ps"];