libbb: provide usleep() fallback implementation

POSIX.1-2008 removed the usleep function, provide a fallback
implementaion using the recommended nanosleep().

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/platform.c b/libbb/platform.c
index 1973451..8d90ca4 100644
--- a/libbb/platform.c
+++ b/libbb/platform.c
@@ -17,6 +17,24 @@
 }
 #endif
 
+#ifndef HAVE_USLEEP
+int FAST_FUNC usleep(unsigned usec)
+{
+	struct timespec ts;
+	ts.tv_sec = usec / 1000000u;
+	ts.tv_nsec = (usec % 1000000u) * 1000u;
+	/*
+	 * If a signal has non-default handler, nanosleep returns early.
+	 * Our version of usleep doesn't return early
+	 * if interrupted by such signals:
+	 *
+	 */
+	while (nanosleep(&ts, &ts) != 0)
+		continue;
+	return 0;
+}
+#endif
+
 #ifndef HAVE_VASPRINTF
 int FAST_FUNC vasprintf(char **string_ptr, const char *format, va_list p)
 {