Work in progress to support a speed factor in SITL

These contains some rough changes trying to get SITL to speed up by a
SPEED_FACTOR.

This platform time code probably requires some more thought and refactor
but this gets a demo at 4x working.

src/include/visibility.h

@@ -73,6 +73,12 @@
 #define system_sleep sleep
 #pragma GCC poison sleep
 
+#include <pthread.h>
+#define system_pthread_cond_timedwait pthread_cond_timedwait
+// We can't poison pthread_cond_timedwait because it seems to be used in the
+// <string> include.
+
+
 #ifdef __cplusplus
 #include <cstdlib>
 #endif