ports-gcc needs to be told to use C++11

https://github.com/milkytracker/MilkyTracker/pull/328

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -23,7 +23,7 @@ cmake_minimum_required(VERSION 3.10)
 project(MilkyTracker)
 
 # Set C++ standard to C++98
-set(CMAKE_CXX_STANDARD 98)
+set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
 # Enable IDE solution folders
