Autogen: Add (CMAKE_)AUTOGEN_ORIGIN_DEPENDS release notes

Help/release/dev/autogen-origin-depends.rst

@@ -0,0 +1,7 @@
+autogen-origin-depends
+----------------------
+
+* A new :variable:`CMAKE_AUTOGEN_ORIGIN_DEPENDS` variable and
+  :prop_tgt:`AUTOGEN_ORIGIN_DEPENDS` target property may be set to enable or
+  disable forwarding of the origin target dependencies to the corresponding
+  ``_autogen`` target.