Switch Minizip to native 64-bit fopen(), ftello(), fseeko().

glibc is basically the only system that uses fopen64(), etc. All other operating systems, and most other Linux C libraries, use fopen() with a 64-bit off_t type. glibc can be configured to use 64-bit off_t with  _FILE_OFFSET_BITS=64 when building applications. This commit does that, because the alternative is having "#if defined __FreeBSD__ || defined __OpenBSD__ || defined __NetBSD__ || defined __DragonFly__ || defined __SUNOS__ || defined __MACOSX__ || ...".

I will attempt to push this change to upstream minizip, but please merge this in the meantime so that users on OpenBSD, etc can build Unvanquished. (But I haven't been able to test on Linux, so make sure it works there first.)