modified subset of zlib-1.2.11 from zlib.net.
note that zlib 1.2.8 has a seek bug that will bite us.
other changes after zlib 1.2.8 may be required as well.
1. contrib/minizip/ioapi.c modifies a conditional adding __CYGWIN__
2. contrib/minizip/ioapi.h modifies a conditional adding __FreeBSD__
See the corresponding .patch files.
