The optimization you are talking about is called wait morphing. Linux futexes have support for this optimization, and glibc pthread_cond_signal did support it at one point. I'm not sure wether they still do though.

Unless you are writing hard realtime systems where you can make use of the strict scheduling guarantees of SCHED_FIFO, signaling while holding the lock is never necessary nor sufficient for correctness. The waiter must be checking the condition in a loop anyway.