Modules/_ctypes/callproc.c:PyCArg_repr() uses sprintf(%qd, long long),
which is a GNU (and more?) extension.  ISO C99 says %lld.

Instead, use "%" PY_FORMAT_LONG_LONG "d" from pyconfig.h/pyport.h.
Kills off #ifdef MS_WIN32 too.  If Python must support C libraries
that handle %qd but not %lld, configure.in seems the right place.

Index: ./Modules/_ctypes/callproc.c
@@ -468,8 +468,3 @@ PyCArg_repr(PyCArgObject *self)
     case 'Q':
-        sprintf(buffer,
-#ifdef MS_WIN32
-            "<cparam '%c' (%I64d)>",
-#else
-            "<cparam '%c' (%qd)>",
-#endif
+        sprintf(buffer, "<cparam '%c' (%" PY_FORMAT_LONG_LONG "d)>",
             self->tag, self->value.q);

pyport.h tests (defined(MS_WIN64) || defined(MS_WINDOWS)) instead of
#ifdef MS_WIN32 for when to use %I64d. I assume that's more up to date.

I agree that this violates C99, but is this actually causing any real world problems with the platforms Python supports?  If so, then we need a test case.  This seems low priority

Objects/stringobject.c already does something similar:
                else if (longlongflag)
                    sprintf(s, "%" PY_FORMAT_LONG_LONG "d",
                        va_arg(vargs, PY_LONG_LONG));
so it makes sense to do the same in Modules/_ctypes/callproc.c.

OpenVMS needs %lld or "%" PY_FORMAT_LONG_LONG "d" in order to build the _ctypes module.

As VMS is not supported I doubt that people here are too bothered about what is needed to build the _ctypes module on it.

I've created a PR for this issue.

New changeset 062cbb67726f26794b1b461853e40696b4a0b220 by Serhiy Storchaka (Zackery Spytz) in branch 'master':
bpo-10320: Replace nonstandard sprintf() length modifier in ctypes' PyCArg_repr(). (GH-10853)
https://github.com/python/cpython/commit/062cbb67726f26794b1b461853e40696b4a0b220

New changeset a9f435e5d856fb62516b70a78217e40b90bec233 by Miss Islington (bot) in branch '3.7':
bpo-10320: Replace nonstandard sprintf() length modifier in ctypes' PyCArg_repr(). (GH-10853)
https://github.com/python/cpython/commit/a9f435e5d856fb62516b70a78217e40b90bec233

New changeset f65ede3f8fa08493facf48177540d0ec26e59560 by Miss Islington (bot) in branch '3.6':
bpo-10320: Replace nonstandard sprintf() length modifier in ctypes' PyCArg_repr(). (GH-10853)
https://github.com/python/cpython/commit/f65ede3f8fa08493facf48177540d0ec26e59560

New changeset 53e2248a94cd89e65326c5cfd400f74a88552d8c by Miss Islington (bot) in branch '2.7':
bpo-10320: Replace nonstandard sprintf() length modifier in ctypes' PyCArg_repr(). (GH-10853)
https://github.com/python/cpython/commit/53e2248a94cd89e65326c5cfd400f74a88552d8c

New changeset 838645dc4191c4109e2b300cf9ed9d481b55509f by Serhiy Storchaka (Zackery Spytz) in branch '2.7':
bpo-10320: Use PY_FORMAT_LONG_LONG in ctypes' PyCArg_repr(). (GH-11230)
https://github.com/python/cpython/commit/838645dc4191c4109e2b300cf9ed9d481b55509f