Improve threading on Solaris, according to SF patch #460269, submitted
by bbrox@bbrox.org / lionel.ulmer@free.fr.

This adds a configure check and if all goes well turns on the
PTHREAD_SCOPE_SYSTEM thread attribute for new threads.

This should remove the need to add tiny sleeps at the start of threads
to allow other threads to be scheduled.
diff --git a/acconfig.h b/acconfig.h
index 69dc3bf..ded68bb 100644
--- a/acconfig.h
+++ b/acconfig.h
@@ -155,6 +155,9 @@
 /* The number of bytes in a pthread_t. */
 #undef SIZEOF_PTHREAD_T
 
+/* Defined if PTHREAD_SCOPE_SYSTEM supported. */
+#undef PTHREAD_SYSTEM_SCHED_SUPPORTED
+
 /* sizeof(void *) */
 #undef SIZEOF_VOID_P