Newish version of MacOS's time.h already `typedef`s `clockid_t` but doesn't seem to `#define` anything indicating such; as a workaround assume that `#ifdef CLOCK_MONOTONIC` on a Mac means that `clockid_t` has been `typedef`'d.
Reattach the licenses for the compat stuff to the source.
Only build/link the compat stuff when it's needed.
Rename compat/util.h so it's not confused with util.h.
Move all the platform specific defines inside compat/compat.h.