CMake: Prefer latest C++ standard with min C++11

Project/CMake/CMakeLists.txt

@@ -166,11 +166,10 @@ set_target_properties(zen PROPERTIES
   VERSION ${ZenLib_VERSION}
   SOVERSION ${ZenLib_MAJOR_VERSION}
   PUBLIC_HEADER "${ZenLib_HDRS}"
-  CXX_STANDARD 20
-  CXX_EXTENSIONS NO
-  CXX_STANDARD_REQUIRED NO
   )
-target_compile_features(zen PRIVATE cxx_std_11)
+
+include(EnableLatestCXX)
+enable_latest_cxx(zen)
 
 install(TARGETS zen EXPORT zen-export
   PUBLIC_HEADER DESTINATION ${INCLUDE_INSTALL_DIR}/ZenLib

Project/CMake/Modules/EnableLatestCXX.cmake

@@ -0,0 +1,26 @@
+function(enable_latest_cxx target)
+
+    set_target_properties(${target} PROPERTIES 
+        CXX_STANDARD_REQUIRED ON
+        CXX_EXTENSIONS OFF
+    )
+
+    # User override
+    if(DEFINED CMAKE_CXX_STANDARD)
+        target_compile_features(${target} PRIVATE cxx_std_${CMAKE_CXX_STANDARD})
+        return()
+    endif()
+
+    # Select highest available from list of supported
+    set(_known_standards 26 23 20 17 14 11)
+    foreach(std ${_known_standards})
+        if("cxx_std_${std}" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+            target_compile_features(${target} PRIVATE cxx_std_${std})
+            message(STATUS "'${target}' is using latest available C++ standard: C++${std}")
+            return()
+        endif()
+    endforeach()
+
+    # Unsupported C++ version
+    message(FATAL_ERROR "Compiler ${CMAKE_CXX_COMPILER_ID} does not support C++11 or newer.")
+endfunction()