Make sure at least C++11 is used

Apparently on macOS we weren't building using c++11, causing a compile error because of a nullptr.
Considering the ubiquitous support for c++11 these days, just enable it.

src/CMakeLists.txt

@@ -2,6 +2,7 @@ cmake_minimum_required (VERSION 3.13)
 
 ### Basic compilation settings
 set (CMAKE_POSITION_INDEPENDENT_CODE TRUE)
+set (CMAKE_CXX_STANDARD 11)
 
 include_directories (
 	${CMAKE_CURRENT_SOURCE_DIR}