haskellPackages.SDL-mixer: fix build on darwin

SDL-mixer's configure script makes some darwin specific assumptions about
the layout of the SDL libraries which don't hold up with the way they are
packaged in nixpkgs. Since the way we are doing this on other platforms
works with the configure script and doesn't significantly differ from the
situation on darwin, we can just replace a string in a string comparison
to ensure the non-darwin code path is taken.

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

@@ -206,6 +206,14 @@ self: super: {
     '' + (drv.postPatch or "");
   });
 
+  # Prevent darwin-specific configuration code path being taken which
+  # doesn't work with nixpkgs' SDL libraries
+  SDL-mixer = overrideCabal super.SDL-mixer (drv: {
+    postPatch = ''
+      substituteInPlace configure --replace xDarwin noDarwinSpecialCasing
+    '' + (drv.postPatch or "");
+  });
+
   # Work around SDL_main.h redefining main to SDL_main
   SDL-ttf = appendPatch super.SDL-ttf ./patches/SDL-ttf-darwin-hsc.patch;