Ensure that usleep() is called with a value under 1000000.

POSIX states that “The usleep() function may fail if: EINVAL The time
interval specified one million or more microseconds.” The mingw-w64
support for usleep() respects this requirement while Linux’s
implementation is fine with any value, so respecting POSIX here
increases portability.
Drop usleep() macro definition for Windows which conflicts with mingw-w64’s compat implementation.

If mowgli.h is #included before the library user #includes unistd.h,
the inclusion of unistd.h from mingw-w64 fails with something like
“unistd.h:35:29: error: expected declaration specifiers or ‘...’
before ‘(’ token”. This fixes the problem by just dropping libmowgli’s
now-unnecessary usleep() compat definition.