I tried to build cpython on cygwin (vista x86).
Core interpretor has built to success, but '_futures' extension module has failed (compile error on Modules/_futuresmodule.c:946 ). It has occured since f8815001a390

part of build log:
$ uname -a
CYGWIN_NT-6.0 masayuki-PC 2.6.0(0.304/5/3) 2016-08-31 14:27 i686 Cygwin
$ ./configure --without-threads --prefix=/opt/py37
$ LANG=C make
(snip)
building '_futures' extension
gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -I./Include -I/opt/py37/include -I. -I/usr/local/include -I/home/masayuki/var/repos/py37-work01/Include -I/home/masayuki/var/repos/py37-work01 -c /home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.c -o build/temp.cygwin-2.6.0-i686-3.7/home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.o
gcc -shared -Wl,--enable-auto-image-base build/temp.cygwin-2.6.0-i686-3.7/home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.o -L. -L/opt/py37/lib -L/usr/local/lib -L. -lpython3.7m -o build/lib.cygwin-2.6.0-i686-3.7/_futures.dll
build/temp.cygwin-2.6.0-i686-3.7/home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.o: In function `new_future_iter':
/home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.c:946: undefined reference to `_PyGC_generation0'
/home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.c:946: undefined reference to `_PyGC_generation0'
/home/masayuki/var/repos/py37-work01/Modules/_futuresmodule.c:946: undefined reference to `_PyGC_generation0'
collect2: error: ld returned 1 exit status
(snip)

I searched declaration for _PyGC_generation0, And it has be found at Include/objimpl.h:259
The found declaration hasn't used PyAPI_DATA macro. Hence, I wrote a patch to wrap declaration by the macro. I has succeeded to build _futures module using the patch.

By the way, I has found similar declarations and defines. I'd like to report them, but Should I create each another issues by places?

Thank you for reporting.

C-API doc [https://docs.python.org/3.7/c-api/gcsupport.html#c._PyObject_GC_TRACK] says:

> A macro version of PyObject_GC_Track(). It should not be used for extension modules.

So simply replacing it to PyObject_GC_Track() may solve the issue.

I need another LGTM from core developer before committing.
Yury, could you see it?  This is one line patch.

Thanks, INADA.
I confirmed your solution, and has succeeded to build _futures extension. Your patch has a little misspell. I fixed it.

revised patch LGTM

Oh! Thanks, While I ran make, I had missed error message somehow.

New changeset a8dd18e375c8 by INADA Naoki in branch '3.6':
Issue #28405: Fix compile error for _futuresmodule.c on Cygwin.
https://hg.python.org/cpython/rev/a8dd18e375c8

New changeset d2a313d13542 by INADA Naoki in branch 'default':
Issue #28405: Fix compile error for _futuresmodule.c on Cygwin.
https://hg.python.org/cpython/rev/d2a313d13542