[Nel] Linux: patch for misc/mutex.cpp (cf. David Mentre post)

[Vincent Caron]

'PTHREAD_MUTEX_FAST_NP' is a GNU extension to Posix threads, either :

- add '#define _GNU_SOURCE' before the '#include <pthread.h>'
- or replace PTHREAD_MUTEX_FAST_NP with PTHREAD_MUTEX_ADAPTIVE_NP

I recommend the second alternative, since it is equivalent (FAST mutex
is currently equivalent to ADAPTIVE mutex) and more portable.