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On Solaris (s10/nitrogen):
% find /usr/include -type f | xargs fgrep -ni AT_FDCWD
/usr/include/sys/fcntl.h:320:#define AT_FDCWD 0xffd19553
(AIX uses -2, FreeBSD uses -100.)
Anyway, that results in:
(cpython@nitrogen:ttypts/10) (Tue/15:34) .. (~/hg/3.x.trent/build/release)
% touch ../../src/Modules/posixmodule.c 
(cpython@nitrogen:ttypts/10) (Tue/15:34) .. (~/hg/3.x.trent/build/release)
% time /usr/ccs/bin/make 
/opt/solarisstudio12.3/bin/cc -DNDEBUG -v -m64 -mt=yes -xbuiltin -xhwcprof -xF -xarch=native -xchip=native -fast -fma=fused -g -xO4 -library=sunperf -I. -I../../src/Include -IInclude -DPy_BUILD_CORE -c ../../src/Modules/posixmodule.c -o Modules/posixmodule.o
"../../src/Modules/posixmodule.c", line 2307: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 2337: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 2386: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 2626: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 2966: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 3198: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 3199: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 3845: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 3989: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 3990: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 4111: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 4267: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 4587: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 7007: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 7092: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 7510: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 8260: warning: initializer does not fit or is out of range: 0xffd19553
"../../src/Modules/posixmodule.c", line 8322: warning: initializer does not fit or is out of range: 0xffd19553
^C
*** Modules/posixmodule.o removed.
/usr/ccs/bin/make 2.79s user 0.28s system 46% cpu 6.598 total
In every case, the affected line is:
 int dir_fd = DEFAULT_DIR_FD;
(DEFAULT_DIR_FD defaults to AT_FDCWD if it's defined.)
Note that Solaris 10 64-bit is LP64, ints are still 32-bit.

Easy fix, cast AT_FDCWD to (int):
% hg diff
diff -r 3a880d640981 Modules/posixmodule.c
--- a/Modules/posixmodule.c Tue Sep 18 07:21:18 2012 +0300
+++ b/Modules/posixmodule.c Tue Sep 18 16:04:58 2012 +0000
@@ -414,7 +414,14 @@
 
 
 #ifdef AT_FDCWD
-#define DEFAULT_DIR_FD AT_FDCWD
+/*
+ * Why the (int) cast? Solaris 10 defines AT_FDCWD as 0xffd19553 (-3041965);
+ * without the int cast, the value gets interpreted as uint (4291925331),
+ * which doesn't play nicely with all the initializer lines in this file that
+ * look like this:
+ * int dir_fd = DEFAULT_DIR_FD;
+ */
+#define DEFAULT_DIR_FD (int)AT_FDCWD
 #else
 #define DEFAULT_DIR_FD (-100)
 #endif
The cast is harmless on other platforms that use an actual integer (rather than a hex representation).
Any objections?

Lgtm. 
Trent Nelson <report@bugs.python.org> wrote:
>
>Trent Nelson added the comment:
>
>Easy fix, cast AT_FDCWD to (int):
>
>
>% hg diff
>diff -r 3a880d640981 Modules/posixmodule.c
>--- a/Modules/posixmodule.c Tue Sep 18 07:21:18 2012 +0300
>+++ b/Modules/posixmodule.c Tue Sep 18 16:04:58 2012 +0000
>@@ -414,7 +414,14 @@
> 
> 
> #ifdef AT_FDCWD
>-#define DEFAULT_DIR_FD AT_FDCWD
>+/*
>+ * Why the (int) cast? Solaris 10 defines AT_FDCWD as 0xffd19553 (-3041965);
>+ * without the int cast, the value gets interpreted as uint (4291925331),
>+ * which doesn't play nicely with all the initializer lines in this file that
>+ * look like this:
>+ * int dir_fd = DEFAULT_DIR_FD;
>+ */
>+#define DEFAULT_DIR_FD (int)AT_FDCWD
> #else
> #define DEFAULT_DIR_FD (-100)
> #endif
>
>
>The cast is harmless on other platforms that use an actual integer (rather than a hex representation).
>
>Any objections?
>
>----------
>
>_______________________________________
>Python tracker <report@bugs.python.org>
><http://bugs.python.org/issue15965>
>_______________________________________

New changeset 974a4cae6094 by Trent Nelson in branch 'default':
#15965: Explicitly cast AT_FDCWD as (int).
http://hg.python.org/cpython/rev/974a4cae6094