glue in working autoconf logic

cf/dispatch.m4

@@ -1,6 +1,20 @@
 
 AC_DEFUN([rk_LIBDISPATCH],[
 
-AM_CONDITIONAL(LIBDISPATCH, test foo = bar)
+AC_CHECK_HEADERS([dispatch/dispatch.h])
+
+AC_FIND_FUNC_NO_LIBS(dispatch_async_f, dispatch,
+[#ifdef HAVE_DISPATCH_DISPATCH_H
+#include <dispatch/dispatch.h>
+#endif],[0,0,0])
+
+if test "$ac_cv_func_dispatch_async_f" = yes ; then
+    AC_DEFINE([HAVE_GCD], 1, [Define if os support gcd.])
+    libdispatch=yes
+else
+    libdispatch=no
+fi
+
+AM_CONDITIONAL(have_gcd, test "$libdispatch" = yes)
 
 ])