Merge pull request #564 from yuriks/reupdate-boost

Update ext-boost commit again which got reverted by #500

externals/boost

@@ -1 +1 @@
-Subproject commit a1afc91d3aaa3da06bdbc13c78613e1466653405
+Subproject commit 728a4d7d1c8b28355544ae829df9c4b5f28373c5