Define _XOPEN_SOURCE in Makefile

So we can depend in headers that its actually set.
(Other headers may be included e.g. before lrc.h,
so that we cant force a value for unistd.h or so anymore.)
lrc.h: Include unistd.h for useconds_t
Provide a usleep-alike wrapper for nanosleep

Since usleep isnt available with XOPEN 700, which is needed for strnlen.

Note: nanosleep needs POSIX >= 199309L, which is implicitly defined by
_XOPEN_SOURCE, see man feature_test_macros(7).

TODO: Can we just move away form useconds_t all and do
all calculations with struct timespec (guess not)..