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

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