Detect more c++ standards and make sure useful message actually appea…

…rs (#635)

* Detect more c++ standards and make sure useful message actually appears

* Order c++ standards numerically

* Use a for loop to detect ROOT c++ standard

* Add a bit more logging

CMakeLists.txt

@@ -88,14 +88,20 @@ endif()
 # ROOT_CXX_STANDARD was introduced in https://github.com/root-project/root/pull/6466
 # before that it's an empty variable so we check if it's any number > 0
 if(NOT DEFINED ROOT_CXX_STANDARD)
+  message(STATUS "ROOT c++ standard not yet available. Determining from compile features")
   get_target_property(ROOT_COMPILE_FEATURES ROOT::Core INTERFACE_COMPILE_FEATURES)
-  if("cxx_std_17" IN_LIST ROOT_COMPILE_FEATURES)
-    set(ROOT_CXX_STANDARD 17)
-  elseif("cxx_std_20" IN_LIST ROOT_COMPILE_FEATURES)
-    set(ROOT_CXX_STANDARD 20)
-  else()
-    message(FATAL_ERROR "ROOT C++ could not be detected")
-  endif()
+  foreach(cxxstd IN ITEMS 20;17;14;11)
+    if ("cxx_std_${cxxstd}" IN_LIST ROOT_COMPILE_FEATURES)
+      set(ROOT_CXX_STANDARD ${cxxstd})
+      break()
+    endif()
+  endforeach()
+endif()
+
+if(NOT DEFINED ROOT_CXX_STANDARD)
+  message(WARNING "ROOT c++ standard could not be detected")
+else()
+  message(STATUS "Determined ROOT c++ standard: " ${ROOT_CXX_STANDARD})
 endif()
 
 if(ROOT_CXX_STANDARD VERSION_LESS 17)