Some ambiguous calls of map::erase have been prevented by adding additional check in locality constructor.

Just struggled with a problem while building with clang-3.5:
![screen shot 2014-12-18 at 8 45 18 pm](https://cloud.githubusercontent.com/assets/5951678/5492918/d166c8f0-86f6-11e4-85f8-95216777bdcc.png)

This issue is described here:
http://cplusplus.github.io/LWG/lwg-active.html#2059


TTI was introduced quite recently, we support older boost releases too (from 1.49 and upwards I believe). You could use MPL_HAS_XXX here instead.