cmake: Use set_target_properties to set C++ standard

This stops dependencies from inheriting a global C++20 standard setting
and instead designates the flycast target alone to be a C++20 project.

CMakeLists.txt

@@ -29,7 +29,7 @@ else()
 	project(flycast)
 endif()
 
-set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 if(NINTENDO_SWITCH)
 	set(CMAKE_CXX_EXTENSIONS ON)
@@ -218,6 +218,10 @@ else()
 	add_executable(${PROJECT_NAME} core/emulator.cpp)
 endif()
 
+set_target_properties(${PROJECT_NAME} PROPERTIES 
+		CMAKE_CXX_STANDARD 20
+		CMAKE_CXX_STANDARD_REQUIRED ON)
+
 if(WINDOWS_STORE)
 	set(USE_OPENGL OFF)
 	set(USE_VULKAN OFF)