Unable to build on Ubuntu 20.04 (focal)

[jelmd]

Trying to build it on Ubuntu 20.04 (focal), but unfortunately it fails with the following errors:

SetupWrapper.hs:84:49: error:
    * Couldn't match type `Distribution.System.Platform -> String'
                     with `[Char]'
      Expected type: String
        Actual type: Distribution.System.Platform -> String
    * Probable cause: `exeExtension' is applied to too few arguments
      In the second argument of `(<.>)', namely `exeExtension'
      In the second argument of `(</>)', namely
        `"setup" <.> exeExtension'
      In the expression: setupDir </> "setup" <.> exeExtension
   |
84 |     setupProgFile    = setupDir </> "setup" <.> exeExtension
   |                                                 ^^^^^^^^^^^^

SetupWrapper.hs:87:24: error:
    * Data constructor not in scope: Version :: [Integer] -> [a0] -> t
    * Perhaps you meant one of these:
        variable `mkVersion' (imported from Distribution.Version),
        variable `version0' (imported from Distribution.Version),
        variable `noVersion' (imported from Distribution.Version)
   |
87 |     useCabalVersion  = Version [1,8] []
   |                        ^^^^^^^

SetupWrapper.hs:113:34: error:
    * Data constructor not in scope:
        PackageName :: [Char] -> PackageName
    * Perhaps you meant one of these:
        variable `packageName' (imported from Distribution.Package),
        variable `mkPackageName' (imported from Distribution.Package),
        variable `unPackageName' (imported from Distribution.Package)
    |
113 |       let cabalDep = Dependency (PackageName "Cabal")
    |                                  ^^^^^^^^^^^

SetupWrapper.hs:117:23: error:
    * Ambiguous type variable `a1' arising from a use of `display'
      prevents the constraint `(Text a1)' from being solved.
      Probable fix: use a type annotation to specify what `a1' should be.
      These potential instances exist:
        instance Text AbiTag -- Defined in `Distribution.Compiler'
        instance Text CompilerFlavor -- Defined in `Distribution.Compiler'
        instance Text CompilerId -- Defined in `Distribution.Compiler'
        ...plus 7 others
        ...plus five instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    * In the first argument of `(++)', namely `display useCabalVersion'
      In the second argument of `(++)', namely
        `display useCabalVersion
           ++ " but no suitable version is installed."'
      In the second argument of `($)', namely
        `"The package requires Cabal library version "
           ++
             display useCabalVersion
               ++ " but no suitable version is installed."'
    |
117 |                    ++ display useCabalVersion
    |                       ^^^^^^^^^^^^^^^^^^^^^^^

SetupWrapper.hs:126:32: error:
    * Ambiguous type variable `a2' arising from a use of `=='
      prevents the constraint `(Eq a2)' from being solved.
      Probable fix: use a type annotation to specify what `a2' should be.
      These potential instances exist:
        instance Eq AbiTag -- Defined in `Distribution.Compiler'
        instance Eq CompilerFlavor -- Defined in `Distribution.Compiler'
        instance Eq CompilerId -- Defined in `Distribution.Compiler'
        ...plus 51 others
        ...plus 73 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    * In the expression:
        majorVersion version == majorVersion cabalVersion
      In an equation for `sameMajorVersion':
          sameMajorVersion
            = majorVersion version == majorVersion cabalVersion
      In an equation for `preference':
          preference version
            = (sameVersion, sameMajorVersion, stableVersion, latestVersion)
            where
                sameVersion = version == cabalVersion
                sameMajorVersion
                  = majorVersion version == majorVersion cabalVersion
                majorVersion = take 2 . versionBranch
                stableVersion
                  = case versionBranch version of
                      (_ : x : _) -> even x
                      _ -> False
                ....
    |
126 |             sameMajorVersion = majorVersion version == majorVersion cabalVersion
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

SetupWrapper.hs:127:41: error:
    Variable not in scope: versionBranch :: a -> [a3]
    |
127 |             majorVersion     = take 2 . versionBranch
    |                                         ^^^^^^^^^^^^^

SetupWrapper.hs:128:37: error:
    Variable not in scope: versionBranch :: Version -> [Integer]
    |
128 |             stableVersion    = case versionBranch version of
    |                                     ^^^^^^^^^^^^^

SetupWrapper.hs:146:45: error:
    * Data constructor not in scope:
        PackageName :: [Char] -> PackageName
    * Perhaps you meant one of these:
        variable `packageName' (imported from Distribution.Package),
        variable `mkPackageName' (imported from Distribution.Package),
        variable `unPackageName' (imported from Distribution.Package)
    |
146 |         let cabalPkgid = PackageIdentifier (PackageName "Cabal") cabalLibVersion
    |                                             ^^^^^^^^^^^

SetupWrapper.hs:180:49: error:
    Variable not in scope: versionBranch :: Version -> [Integer]
    |
180 |             (major1:major2:minor:_) = map show (versionBranch version ++ repeat 0)
    | 

Any hints, how to fix? ^^^^^^^^^^^^^

[jelmd]

Applied patches to build it out of the box:
00_gtk2hs.patch.txt
01_gtk2hs.patch.txt