Add a global variable called __isthreaded that can be tested throughout

libc to determine if locking is required. This is needed in libc
for use with kernel threads, but until a thread is created, we don't
really want to bother locking things. The variable was added here
because the crt code calls exit(main()) so all programs will get the
variable.

lib/libc/stdlib/exit.c

@@ -41,6 +41,15 @@ static char sccsid[] = "@(#)exit.c	8.1 (Berkeley) 6/4/93";
 
 void (*__cleanup)();
 
+/*
+ * This variable is zero until a process has created a thread.
+ * It is used to avoid calling locking functions in libc when they
+ * are not required. By default, libc is intended to be(come)
+ * thread-safe, but without a (significant) penalty to non-threaded
+ * processes.
+ */
+int	__isthreaded	= 0;
+
 /*
  * Exit, flushing stdio buffers if necessary.
  */