[cmake] mention builtin_afterimage as removed

And clarify warning, rather than saying that it is not supported, as someone might think that things that are now ON by default are no longer supported.

cmake/modules/RootBuildOptions.cmake

@@ -392,13 +392,13 @@ if(NOT webgui)
 endif()
 
 #---Removed options------------------------------------------------------------
-foreach(opt afdsmgrd afs alien bonjour castor chirp cxx11 cxx14 cxx17
+foreach(opt afdsmgrd afs alien bonjour builtin_afterimage castor chirp cxx11 cxx14 cxx17
         exceptions geocad gfal glite globus gsl_shared hdfs ios jemalloc krb5
         ldap memstat minuit2 monalisa oracle pyroot-python2 pyroot_legacy
         pythia6 pythia6_nolink python qt qtgsi rfio ruby sapdb srp table
         tcmalloc vmc xproofd)
   if(${opt})
-    message(FATAL_ERROR ">>> Option '${opt}' is no longer supported in ROOT ${ROOT_VERSION}.")
+    message(FATAL_ERROR ">>> '${opt}' is no longer part of ROOT ${ROOT_VERSION} build options.")
   endif()
 endforeach()