diff --git boost/config/platform/macos.hpp boost/config/platform/macos.hpp
index 2780ef9..eb49c6d 100644
--- boost/config/platform/macos.hpp
+++ boost/config/platform/macos.hpp
@@ -73,7 +73,10 @@
 
 // The MP task implementation of Boost Threads aims to replace MP-unsafe
 // parts of the MSL, so we turn on threads unconditionally.
+// Except that MPTasks are no longer supported in Boost.
+#  if defined(BOOST_HAS_PTHREADS)
 #    define BOOST_HAS_THREADS
+#  endif
 
 // The remote call manager depends on this.
 #    define BOOST_BIND_ENABLE_PASCAL