Merge pull request andreasbuhr#31 from andreasbuhr/default_cxx_version

Default CMAKE_CXX_STANDARD to 20

Author: andreasbuhr

CMakeLists.txt

@@ -1,6 +1,11 @@
 cmake_minimum_required(VERSION 3.12)
 project(cppcoro LANGUAGES CXX)
 
+if(NOT "${CMAKE_CXX_STANDARD}")
+  message("C++ version not set. Defaulting to CMAKE_CXX_STANDARD=20")
+  set(CMAKE_CXX_STANDARD 20)
+endif()
+
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)