Fix handling of conditinal properties, broken a recent commit.

1. Don't break where there are several elements in condition.
2. Don't validate <toolset> feature in condition, since we might
   not have initialized all possible toolsets.


[SVN r22964]

tools/build

@@ -1 +1 @@
-Subproject commit e4163c3201c094e267689ce85c7b7ff3afa61818
+Subproject commit 186e2c3da45bb06a391626cb02efaac7b4fa93b4