Based on http://bugs.python.org/issue27490 and http://bugs.python.org/msg271495, here is a patch that makes sure Programs/_freeze_importlib is only built when not cross-compiling.

Here is another possible option. It is still a bit of a hack, because the configure script inserts comments into the makefile, but this is already done e.g. with @EXPORT_MACOSX_DEPLOYMENT_TARGET@. The advantage is we get to keep the filenames of the dependencies in the makefile, so it should be easier to read and maintain. And we get to eliminate the $(cross_compiling) check as well.

In native compiling mode, everything should be as normal. When cross-compiling, the rules for regenerating files should read like

Python/importlib.h: # $(srcdir)/Lib/importlib/_bootstrap.py Programs/_freeze_importlib
	./Programs/_freeze_importlib \
	    $(srcdir)/Lib/importlib/_bootstrap.py Python/importlib.h

Since the Python/importlib.h should already exist, the rule in this form (dependencies commented out) won’t be run, and Make won’t need to build Programs/_freeze_importlib either.

Nice.

FWIW cross-compilation for Android works fine with comment-out-regen.patch.

> In native compiling mode, everything should be as normal. When cross-compiling, the rules for regenerating files should read like
> 
> Python/importlib.h: # $(srcdir)/Lib/importlib/_bootstrap.py Programs/_freeze_importlib
> 	./Programs/_freeze_importlib \
> 	    $(srcdir)/Lib/importlib/_bootstrap.py Python/importlib.h

They do read like this.

+1 on comment-out-regen.patch, makes things much cleaner and removes the shell "if" in the rule body.

Just a small bikeshed issue: Instead of COMMENT_REGEN, maybe call it "CROSS_COMPILE_COMMENT" or "GENERATED_COMMENT" or "COMMENT_IF_CROSS" or somesuch? This way, might be easier to read/understand the makefile rules ("COMMENT_IF_CROSS" -> "a comment character will be inserted here if cross-compiling")?

Thanks for the feedback. I did wonder about the name. Perhaps I will go with GENERATED_COMMENT:

GENERATED_COMMENT='#'

Python/importlib_external.h: @GENERATED_COMMENT@ $(srcdir)/Lib/importlib/_bootstrap_external.py Programs/_freeze_importlib

New changeset bc677cb34889 by Martin Panter in branch '3.5':
Issue #27641: Comment out regeneration rules when cross compiling
https://hg.python.org/cpython/rev/bc677cb34889

New changeset fc034d3607a8 by Martin Panter in branch 'default':
Issue #27641: Merge cross-compiling improvement from 3.5
https://hg.python.org/cpython/rev/fc034d3607a8

New changeset e3757f3e1848 by Martin Panter in branch '2.7':
Issue #27641: Comment out regeneration rules when cross compiling
https://hg.python.org/cpython/rev/e3757f3e1848

_freeze_import is only in Python 3, but I backported my final change because it is more general and also affects pgen.