clucene-core: make darwin-related patch conditional

This relates to pull request #6137, and was made at request of the
submitter.

pkgs/development/libraries/clucene-core/2.x.nix

···
     [ ./Fix-pkgconfig-file-by-adding-clucene-shared-library.patch
       ./Fixing_ZLIB_configuration_in_shared_CMakeLists.patch
       ./Install-contribs-lib.patch
-      ./fix-darwin.patch
-    ];
+    ] ++ stdenv.lib.optionals stdenv.isDarwin [ ./fix-darwin.patch ];
 
   postInstall = stdenv.lib.optionalString stdenv.isDarwin ''
     install_name_tool -change libclucene-shared.1.dylib \