> However, experience tells that systems can break in surprising ways
> if the system headers are compiled with different defines.

This is indeed a reasonable concern (for which the best solution is
dropping the defines in the Python compile).

> I do feel this restrictiveness of the header files (wrt. C99) is
> arbitrary, and has no use.

Unfortunately, neither you, I, nor Sun can do anything at all about this
fact. I've filed an RFE for you:

http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6806390
feature_tests.h could be laxer about C99

(It'll take a while to be visible externally.)

Even if this happens, though, it's strictly a workaround: it is still a
bug that Python sets these defines on Solaris: they do not mean what the
Python build thinks they mean. It's effectively a Linux-ism.

> setting _XOPEN_SOURCE had worked fine since Solaris 7.

The particular change we're talking about (C99) went back in 2003:

http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=4760794
4760794 UNIX03: sys/feature_tests.h updates required to support SUSv3
and c99

so Python has been broken by this for all of the Solaris 10 lifecycle.