mingw.c: ignore EACCES from mkdir if directory exists

Microsoft Windows has a strange issue with access permissions such that
mkdir will sometimes return EACESS for an existing directory.  This is
mentioned here:

   http://www.apijunkie.com/APIJunkie/blog/post/2009/12/22/_mkdir-C-runtime-library-function-might-return-unexpected-error-values.aspx

This was causing mkdir -p to fail on a particular machine.  Ignoring the
EACCES error if the directory exists fixes the problem.