meson fixes to build VLC with clang-cl

This is needed in addition to !3048 (merged) and !3065 (merged).

• A dummy unistd.h is included so that all the code using that header doesn't need to be changed.
• dirent.h usage is disabled but it wouldn't be used for compiled code anyway.
• tests not building on Windows are fixed disabled (tests are always compiled even if not used)