dfhack: fix ruby plugin

dfhack expects an unversioned libruby.so to be present in the hack
subdirectory for ruby plugins to function. The glob is necessary,
because the ruby package's library names are formatted as
ruby-${version}.so for whatever reason. The glob makes it possible for
ruby to update without manual intervention being needed here.

pkgs/games/dwarf-fortress/dfhack/default.nix

@@ -1,5 +1,5 @@
 { stdenv, buildEnv, lib, fetchFromGitHub, cmake, writeScriptBin
-, perl, XMLLibXML, XMLLibXSLT, zlib
+, perl, XMLLibXML, XMLLibXSLT, zlib, ruby
 , enableStoneSense ? false,  allegro5, libGLU, libGL
 , enableTWBT ? true, twbt
 , SDL
@@ -140,6 +140,12 @@ let
     cmakeFlags = [ "-DDFHACK_BUILD_ARCH=${arch}" "-DDOWNLOAD_RUBY=OFF" ]
               ++ lib.optionals enableStoneSense [ "-DBUILD_STONESENSE=ON" "-DSTONESENSE_INTERNAL_SO=OFF" ];
 
+    # dfhack expects an unversioned libruby.so to be present in the hack
+    # subdirectory for ruby plugins to function.
+    postInstall = ''
+      ln -s ${ruby}/lib/libruby-*.so $out/hack/libruby.so
+    '';
+
     enableParallelBuilding = true;
   };
 in