bpo-23404: `make regen-all` What's New entry (#2128)

While the build changes won't affect most users,
they may affect redistributors and folks building
their own Python binaries from source.

Doc/whatsnew/3.6.rst

@@ -2306,3 +2306,39 @@ There have been several major changes to the :term:`bytecode` in Python 3.6.
 * The new :opcode:`SETUP_ANNOTATIONS` and :opcode:`STORE_ANNOTATION` opcodes
   have been added to support the new :term:`variable annotation` syntax.
   (Contributed by Ivan Levkivskyi in :issue:`27985`.)
+
+
+Notable changes in Python 3.6.2
+===============================
+
+New ``make regen-all`` build target
+-----------------------------------
+
+To simplify cross-compilation, and to ensure that CPython can reliably be
+compiled without requiring an existing version of Python to already be
+available, the autotools-based build system no longer attempts to implicitly
+recompile generated files based on file modification times.
+
+Instead, a new ``make regen-all`` command has been added to force regeneration
+of these files when desired (e.g. after an initial version of Python has
+already been built based on the pregenerated versions).
+
+More selective regeneration targets are also defined - see
+:source:`Makefile.pre.in` for details.
+
+(Contributed by Victor Stinner in :issue:`23404`.)
+
+.. versionadded:: 3.6.2
+
+
+Removal of ``make touch`` build target
+--------------------------------------
+
+The ``make touch`` build target previously used to request implicit regeneration
+of generated files by updating their modification times has been removed.
+
+It has been replaced by the new ``make regen-all`` target.
+
+(Contributed by Victor Stinner in :issue:`23404`.)
+
+.. versionchanged:: 3.6.2