[PATCH] Replace the Execution Engine's mutex with std::recursive_mutex

[Vadim Chugunov]

FYI - this commit broke LLVM build using [[ http://stackoverflow.com/questions/13212342/whats-the-difference-between-thread-posixs-and-thread-win32-in-gcc-port-of-windo | win32 threads ]] flavor of the mingw toolchain.  I am getting [[ http://stackoverflow.com/questions/14191566/c-mutex-in-namespace-std-does-not-name-a-type | error: 'recursive_mutex' in namespace 'std' does not name a type ]].  
Not sure if this would be considered a problem for LLVM...

http://reviews.llvm.org/D4196