[RFC] remove gettimeofday() usage

Ping @equalsraf. When this is merged, you'll probably be able to excise a hack.
That's also a really good catch. See this:

```c
#if defined(UNIX) && defined(HAVE_GETTIMEOFDAY) && defined(HAVE_SYS_TIME_H)
# define CHECK_DOUBLE_CLICK 1  /* Checking for double clicks ourselves. */
#endif
```

As far as I can see, that would mean that the `DOUBLE_CLICK` code would no longer be activated after the commit in which I removed the cmake detection.

I also don't know why it asks for `defined(UNIX)` because in the `:help` it says that this also works for Windows. Did it get lost in the vim/neovim split?