third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/cmake/libprotobuf.cmake

 set(libprotobuf_files
   ${protobuf_source_dir}/src/google/protobuf/any.cc
   ${protobuf_source_dir}/src/google/protobuf/any.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/api.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/importer.cc
   ${protobuf_source_dir}/src/google/protobuf/compiler/parser.cc
   ${protobuf_source_dir}/src/google/protobuf/descriptor.cc
   ${protobuf_source_dir}/src/google/protobuf/descriptor.pb.cc
   ${protobuf_source_dir}/src/google/protobuf/descriptor_database.cc
   ${protobuf_source_dir}/src/google/protobuf/duration.pb.cc
@@ skipping 38 matching lines @@
   ${protobuf_source_dir}/src/google/protobuf/util/json_util.cc
   ${protobuf_source_dir}/src/google/protobuf/util/message_differencer.cc
   ${protobuf_source_dir}/src/google/protobuf/util/time_util.cc
   ${protobuf_source_dir}/src/google/protobuf/util/type_resolver_util.cc
   ${protobuf_source_dir}/src/google/protobuf/wire_format.cc
   ${protobuf_source_dir}/src/google/protobuf/wrappers.pb.cc
 )
 
 add_library(libprotobuf ${protobuf_SHARED_OR_STATIC}
   ${libprotobuf_lite_files} ${libprotobuf_files})
-target_link_libraries(libprotobuf ${CMAKE_THREAD_LIBS_INIT} ${ZLIB_LIBRARIES})
+target_link_libraries(libprotobuf ${CMAKE_THREAD_LIBS_INIT})
+if(protobuf_WITH_ZLIB)
+    target_link_libraries(libprotobuf ${ZLIB_LIBRARIES})
+endif()
 target_include_directories(libprotobuf PUBLIC ${protobuf_source_dir}/src)
 if(MSVC AND protobuf_BUILD_SHARED_LIBS)
   target_compile_definitions(libprotobuf
     PUBLIC  PROTOBUF_USE_DLLS
     PRIVATE LIBPROTOBUF_EXPORTS)
 endif()
 set_target_properties(libprotobuf PROPERTIES
     OUTPUT_NAME ${LIB_PREFIX}protobuf
     DEBUG_POSTFIX "${protobuf_DEBUG_POSTFIX}")