Build --static on windows

This small changes ensure that when building under mingw (with conda or a plain mingw) the resulting _graphillion.pyd library does not have extra dependencies that are MinGW-specific such as libstdc++ or libgcc. This allows to build further wheels and eggs that run out of the box. The (minor) downside is that the shared library is about twice bigger (1MB vs 500KB).
Merge branch 'pombredanne-patch-1' into v0.96rc