threads: use static error check mutex initializer if available

This changes the condition to use the static error check mutex
initializer.  If it is not available, use the default static mutex
initializer.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/threads.h

@@ -29,7 +29,7 @@
 #include <pthread.h>
 #define SSH_MUTEX pthread_mutex_t
 
-#if defined _GNU_SOURCE
+#if defined(PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP)
 #define SSH_MUTEX_STATIC_INIT PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP
 #else
 #define SSH_MUTEX_STATIC_INIT PTHREAD_MUTEX_INITIALIZER