Issue python#14605 and python#14642:

Issue a warning in case Python\importlib.h needs to be rebuilt,
but there's no Python interpreter around to freeze the bootstrap
script.

Lib/importlib/_bootstrap.py

@@ -6,6 +6,13 @@
 work. One should use importlib as the public-facing version of this module.
 
 """
+#
+# IMPORTANT: Whenever making changes to this module, be sure to run
+# a top-level make in order to get the frozen version of the module
+# update. Not doing so, will result in the Makefile to fail for
+# all others who don't have a ./python around to freeze the module
+# in the early stages of compilation.
+#
 
 # See importlib._setup() for what is injected into the global namespace.