Add a function to detect if we have async or not

Add the ASYNC_is_capable() function and use it in speed.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/async/arch/async_posix.c

@@ -60,11 +60,15 @@
 
 #define STACKSIZE       32768
 
+int ASYNC_is_capable(void)
+{
+    return 1;
+}
+
 void async_local_cleanup(void)
 {
 }
 
-
 int async_fibre_makecontext(async_fibre *fibre)
 {
     fibre->env_init = 0;