Only use process shared mutexes on support platforms

On FreeBSD, suggest the use of threads instead.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/os/os-solaris.h b/os/os-solaris.h
index 51c0ff4..35148d2 100644
--- a/os/os-solaris.h
+++ b/os/os-solaris.h
@@ -10,6 +10,7 @@
 #define FIO_HAVE_FALLOCATE
 #define FIO_HAVE_POSIXAIO_FSYNC
 #define FIO_HAVE_CPU_AFFINITY
+#define FIO_HAVE_PSHARED_MUTEX
 
 #define OS_MAP_ANON		MAP_ANON
 #define OS_RAND_MAX		2147483648UL