POSIX specification: http://www.opengroup.org/susv3xsh/usleep.html
Gnulib module: usleep
Portability problems fixed by Gnulib:
usleeprejects attempts to sleep longer than 1 second, as allowed by POSIX: mingw.
nanosleepif better resolution is needed: IRIX 5.3, Solaris 2.4, older mingw, MSVC 9, BeOS.
Portability problems not fixed by Gnulib:
usleepfunction may interfere with the program's use of the
SIGALRMsignal. On Linux, it doesn't; on other platforms, it may.