ABI incompatibilities with MinGW GCC 4.7.0

See http://sourceforge.net/mailarchive/message.php?msg_id=29376223 and
the following discussion (which is still unfolding) for the details.

The upshot of this, AFAIU, is that the MinGW GCC 4.7.0 should NOT be
used for building Emacs on Windows with any of the optional libraries,
such as image libraries, GnuTLS, libxml2, etc., because _all_ of those
libraries were compiled with versions of GCC before 4.7.0, and are now
ABI incompatible with code compiled by 4.7.0.

(Actually, you cannot safely use the MinGW GCC 4.7.0 for building
Emacs on Windows at all for now, because (a) there's no MinGW runtime
available that is compatible with the new ABI, and (b) you must link
with libxpm.dll, which was compiled by an older GCC.)

I sincerely hope that this incompatibility will either be reverted or
turns out as an insignificant one, because otherwise we will be facing
a lot of subtle and hard to reproduce bugs in the Windows build.  The
only alternative, if this issue is not resolved, is to not upgrade to
GCC 4.7.