Work around a bug in the VC++10 standard library. Hopefully fixes bug 3594; regression tests will tell. This may need to be updated if subsequent releases have the same bug.

[SVN r59734]

libs/property_tree

@@ -1 +1 @@
-Subproject commit d660aa0ba04fa7b323d1defbd5c55fb326d5615b
+Subproject commit 1abc021f161c501e1f9bfe4521629864959acd87