cmake: when webui cannot be build - just disable it

Do not produce fatal error, which may happen if try to compile with
-Dall=ON option

cmake/modules/RootBuildOptions.cmake

@@ -229,6 +229,7 @@ if(all)
  set(r_defvalue ON)
  set(roofit_defvalue ON)
  set(root7_defvalue ON)
+ set(webui_defvalue ON)
  set(shadowpw_defvalue ON)
  set(table_defvalue ON)
  set(unuran_defvalue ON)
@@ -302,8 +303,8 @@ ROOT_APPLY_OPTIONS()
 
 #---check if webui can be build-------------------------------
 if(webui)
-  if(CMAKE_CXX_STANDARD EQUAL 11)
-    message(FATAL_ERROR "WebUI requires C++14 or higher")
+  if(NOT CMAKE_CXX_STANDARD GREATER 11)
+    set(webui OFF CACHE BOOL "(WebUI requires at least C++14)" FORCE)
   elseif(NOT http)
     set(http ON CACHE BOOL "(Enabled since it's needed by webui)" FORCE)
   endif()