Merge pull request #192 from eregon/mvm

Make environment variables thread-safe
Added the debugEntry op codes, and let the StackTrace use them.

This is mainly to make `ozc -g` work and reports more accurate line numbers
on crash.
Merge pull request #193 from kennytm/debug-entry

Make `ozc -g` and stack traces work
Prefer boost::mutex to std::mutex as they are not implemented in MinGW

* We are in 2014 and it is C++11, sigh.
* Also fixes a mismatch.
Merge pull request #194 from eregon/boost_mutex

Prefer boost::mutex to std::mutex as they are not implemented in MinGW
Ensure to release the VM list mutex before destroying the environment