[Mingw-cross-env-list] updated mingwrt

[Mark Brand]

Hi,

I updated mingwrt to version  3.20.

http://hg.savannah.gnu.org/hgweb/mingw-cross-env/rev/50cb486c5e93

It would be good to have an extra set of eyes go over the following:

3.20 seems to fix the float.h problem, so our patch is no longer needed.

Also, ssize_t is now provided by mingwrt, which confuses libdnet and 
libarchive. I added minor patches for these.

All packages are building fine for me.

regards,

Mark