[Pixman] MinGW thread local storage

[Søren Sandmann]

Hi,

The patches below are supposed to fix the thread local storage issue
on MinGW 32. Apparently, __thread is "supported" in the latest GCC,
but doesn't actually work. The first patch here simply disables
__thread if MinGW32 is detected.

The second patch is based on Tor's code from here:

    http://lists.freedesktop.org/archives/pixman/2010-April/000160.html

Sven Goericke tested a tarball with these patches applied, so it
apparently works, but more testing from people using MinGW32 would be
appreciated.

Also, I would still like to know if which pixman identifiers the
<windows.h> file is clashing with; maybe we can just rename those.



Soren