add deprecation message

mention deprecation in release notes

README/ReleaseNotes/v632/index.md

@@ -45,6 +45,7 @@ The following people have contributed to this new version:
   Please use the RooDataSet constructors that take RooFit command arguments instead
 - ROOT does not longer support Python 2. The minimum required Python version to build ROOT is 3.8.
 - Support for wildcard imports like `from ROOT import *` is dropped from PyROOT
+- Support for external (ie. non-builtin) libAfterImage is now deprecated and it will be removed in next release 6.34.
 
 ## Core Libraries
 

cmake/modules/RootBuildOptions.cmake

@@ -80,7 +80,7 @@ endfunction()
 ROOT_BUILD_OPTION(arrow OFF "Enable support for Apache Arrow")
 ROOT_BUILD_OPTION(asimage ON "Enable support for image processing via libAfterImage")
 ROOT_BUILD_OPTION(asserts OFF "Enable asserts (defaults to ON for CMAKE_BUILD_TYPE=Debug and/or dev=ON)")
-ROOT_BUILD_OPTION(builtin_afterimage ON "Build bundled copy of libAfterImage")
+ROOT_BUILD_OPTION(builtin_afterimage ON "Build bundled copy of libAfterImage (flag is deprecated, this should be always enabled)")
 ROOT_BUILD_OPTION(builtin_cfitsio OFF "Build CFITSIO internally (requires network)")
 ROOT_BUILD_OPTION(builtin_clang ON "Build bundled copy of Clang")
 ROOT_BUILD_OPTION(builtin_cling ON "Build bundled copy of Cling. Only build with an external cling if you know what you are doing: associating ROOT commits with cling commits is tricky.")
@@ -419,6 +419,12 @@ foreach(opt cxxmodules exceptions oracle pythia6 pythia6_nolink minuit2)
   endif()
 endforeach()
 
+foreach(opt builtin_afterimage)
+  if(NOT ${opt})
+    message(DEPRECATION ">>> Option '${opt}' is deprecated and will be removed in the next release of ROOT. It should always be ON. Please contact root-dev@cern.ch should you still need it.")
+  endif()
+endforeach()
+
 foreach(opt minuit2_omp minuit2_mpi)
   if(${opt})
       message(WARNING "The option '${opt}' can only be used to minimise thread-safe functions in Minuit2. It cannot be used for Histogram/Graph fitting and for RooFit. If you want to use Minuit2 with OpenMP or MPI support, it is better to build Minuit2 as a standalone library.")