Build with the thebes gfx backend by default on Windows. bug 323923. r=vlad sr=roc.

configure.in

@@ -1741,7 +1741,7 @@ case "$target" in
     AC_DEFINE(NEW_H, <new>)
     AC_DEFINE(WIN32_LEAN_AND_MEAN)
     TARGET_MD_ARCH=win32
-    _PLATFORM_DEFAULT_TOOLKIT='windows'
+    _PLATFORM_DEFAULT_TOOLKIT='cairo-windows'
     BIN_SUFFIX='.exe'
     USE_SHORT_LIBNAME=1
     MOZ_ENABLE_POSTSCRIPT=