CMakeLists: Build library with -fvisibility=protected

This reduces the number of dynamic relocs by nearly 50%. Unfortunately
doing so causes errors like:
 /usr/bin/ld: CMakeFiles/dbus-asio.dir/src/dbus_auth.cpp.o: relocation R_X86_64_PC32 against protected symbol `_ZN4DBus22AuthenticationProtocol4onOKERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE' can not be used when making a shared object

when linking with ld.bfd. Let's use gold instead, which doesn't suffer
from that problem.

CMakeLists.txt

@@ -2,6 +2,9 @@ cmake_minimum_required(VERSION 3.7.1)
 
 project(dbus-asio VERSION 1.0.0)
 
+set(CMAKE_CXX_VISIBILITY_PRESET protected)
+set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -fuse-ld=gold")
+
 set(HEADER_FILES
 	src/dbus_auth.h
 	src/dbus.h