Comment by Daviey

It means it has a dependency on X11.

  $ go install github.com/ramonvermeulen/whosthere@latest
  # golang.design/x/clipboard
  clipboard_linux.c:14:10: fatal error: X11/Xlib.h: No such file or directory
    14 | #include <X11/Xlib.h>
       |          ^~~~~~~~~~~~
  compilation terminated.