[Mingw-cross-env-list] gcc 4.6.0

[Mark Brand]

Hi,

GCC 4.6.0 seems to be working pretty well.
http://hg.savannah.gnu.org/hgweb/mingw-cross-env/rev/e73e39579ac1

Several packages needed some mostly trivial adjustments which are in the 
subsequent changesets. Review is welcome.

There is still some trouble with geos. The definitions for size_t and 
NULL are missing somehow. There's probably a simple solution, but I 
haven't found it yet. If anybody would like to take a stab at this, 
please feel free to do so. Once geos works, we should check that gdal 
and openscenegraph are okay.

Mark