Fix MXE build of libzip

[grendello]

Commit fa9f62a failed to update the MXE build of the library
and so it was still being built as a static archive (which is the
new libzip default) and caused build failures when building
libzip executables.

This commit makes MXE build a shared version of the library and,
hopefully, fixes the issue.

[jonpryor]

macOS+xbuild still fails:

Linking C executable hole.exe
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x77): undefined reference to `__imp_zip_error_init'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x9b): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0xcf): undefined reference to `__imp_zip_error_fini'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0xf5): undefined reference to `__imp_zip_error_init'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x106): undefined reference to `__imp_zip_source_file_create'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x11d): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x151): undefined reference to `__imp_zip_error_fini'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x17d): undefined reference to `__imp_zip_source_open'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x18e): undefined reference to `__imp_zip_source_begin_write'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x1a2): undefined reference to `__imp_zip_source_read'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x1a9): undefined reference to `__imp_zip_source_write'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x1e0): undefined reference to `__imp_zip_source_close'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x1e9): undefined reference to `__imp_zip_source_commit_write'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x20e): undefined reference to `__imp_zip_source_error'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x217): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x246): undefined reference to `__imp_zip_source_close'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x24f): undefined reference to `__imp_zip_source_rollback_write'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x25a): undefined reference to `__imp_zip_source_error'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x263): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x28f): undefined reference to `__imp_zip_source_error'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x298): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x2cf): undefined reference to `__imp_zip_source_error'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x2d8): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x307): undefined reference to `__imp_zip_source_close'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x318): undefined reference to `__imp_zip_source_error'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x321): undefined reference to `__imp_zip_error_strerror'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text+0x350): undefined reference to `__imp_zip_source_rollback_write'
CMakeFiles/hole.dir/objects.a(hole.c.obj):hole.c:(.text.startup+0xb3): undefined reference to `__imp_zip_source_free'
CMakeFiles/hole.dir/objects.a(source_hole.c.obj):source_hole.c:(.text+0x95): undefined reference to `__imp_zip_error_to_data'
CMakeFiles/hole.dir/objects.a(source_hole.c.obj):source_hole.c:(.text+0xa9): undefined reference to `__imp_zip_error_fini'
CMakeFiles/hole.dir/objects.a(source_hole.c.obj):source_hole.c:(.text+0x169): undefined reference to `__imp_zip_source_seek_compute_offset'
...

What's weird is that this fails after 3h50min: libzip and company are built for the Debug configuration, but not for the Release configuration.

[jonpryor]

In the meantime, I'm reverting PR #863 so that we get get master green again, and resume merging PRs.

[grendello]

New PR opened which combines both *nix and Windows changes (and hopefully fixes the build): #874