Abandon the old struct timeval and gettimeofday(). We now use the new
struct timespec and clock_gettime().
struct timespec has a nanosecond resolution, but we lower the resolution
to keep only milliseconds. The code is inspired from the Timestamp class
of the owlps-positioning.
Symbolic link libowlps.so is required to dynamically compile, if
"-lowlps" is used instead of "../../libowlps/libowlps.so.1.0" (which is
now the case in the listener's makefiles, and will be soon for the
others too).