This change concerne the usage of the semaphore object instead the variable condition...

This change concerne the usage of the semaphore object instead the variable condition to synchronize the msg processes during a simulation.
The xbt_os_sem_t type represents a semaphore and you' ll find its declaration in the header xbt_os_thread.h and its implementation in the file xbt_os_thread.c.
The file portable.h is modified to declare the function gettimeofday() used by many several other functions such the function xbt_os_sem_timedwait() declared in
the header xbt_os_thread.h. The context structure declared in the header context_private.h is modified to use the semaphore instead the condition variable.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@4838 48e7efb5-ca39-0410-a469-dd3cf9ba447f