win32: allow building tests with Meson

alarm() is only used by tests in VLC. It doesn't exist on Windows but we can define it to do nothing and let the tests build on Windows. The define means we need to rename variables named "alarm" in the code. Luckily there is none in contribs headers.

Running the tests still mostly fails because the libvlcore and libvlc DLLs are not in the PATH.