build_system: make ENTT_CXX_STD a cached string variable (not used yet, see #751)

CMakeLists.txt

@@ -175,6 +175,7 @@ if(ENTT_BUILD_TESTING)
     option(ENTT_BUILD_SNAPSHOT "Build snapshot test with Cereal." OFF)
 
     set(ENTT_ID_TYPE std::uint32_t CACHE STRING "Type of identifiers to use for the tests")
+    set(ENTT_CXX_STD cxx_std_17 CACHE STRING "C++ standard revision to use for the tests")
 
     include(CTest)
     enable_testing()

test/CMakeLists.txt

@@ -38,6 +38,7 @@ add_compile_options($<TARGET_PROPERTY:EnTT,INTERFACE_COMPILE_OPTIONS>)
 
 function(SETUP_TARGET TARGET_NAME)
     set_target_properties(${TARGET_NAME} PROPERTIES CXX_EXTENSIONS OFF)
+    target_compile_features(${TARGET_NAME} PRIVATE ${ENTT_CXX_STD})
     target_link_libraries(${TARGET_NAME} PRIVATE EnTT)
 
     if(MSVC)