Make it possible to compile with gcc4.6

Makefile

examples/pubsub/publisher_test.cc

examples/pubsub/subscriber_test.cc

include/grpc++/async_unary_call.h

include/grpc++/client_context.h

include/grpc++/config.h

include/grpc++/credentials.h

include/grpc++/impl/call.h

include/grpc++/impl/rpc_service_method.h

include/grpc++/impl/service_type.h

include/grpc++/server.h

include/grpc++/server_builder.h

include/grpc++/server_context.h

include/grpc++/server_credentials.h

include/grpc++/stream.h

src/compiler/cpp_generator.cc

src/compiler/python_generator.cc

src/compiler/python_plugin.cc

src/compiler/ruby_plugin.cc

src/cpp/client/channel.h