Mention SDL2/SDL3 changes (#24306) in ChangeLog. NFC

Author: sbc100

ChangeLog.md

@@ -32,6 +32,10 @@ See docs/process.md for more on how version tagging works.
 
 4.0.9 - 05/19/25
 ----------------
+- cmake will not longer detect SDL2 or SDL3 as being present until they are
+  installed in the sysroot.  This means that they now need to be installed,
+  either indirectly by running (e.g. by running `emcc -sUSE_SDL=2`) or dirctly
+  (e.g. by running `./embuilder build sdl2`). (#24306)
 - libunwind was updated to LLVM 20.1.4. (#24251)
 - When using cmake the EMSCRIPTEN_FORCE_COMPILERS setting was reverted to
   being on by default due to issues that were found with disabling it. (#24223)