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

[Zachary Turner]

+llvmdev.

I find this pretty surprising.  Actually, we already use std::mutex and
std::recursive_mutex in clang, lld, and other llvm projects, it's just a
coincidence that it hadn't been introduced into LLVM until my commits.

I'm not sure what the right thing to do here is.  If I understand
correctly, it seems like in order to encounter this, a) you must be using
GCC, b) you must be using the MinGW flavor of GCC, and c) you must be using
the threads-win32 flavor of this toolchain.   Only if all 3 of those are
true, then std::mutex and std::recursive_mutex don't exist.

Anybody else have thoughts on whether this necessitates reverting the mutex
changes, or whether this toolchain configuration should be supported?

http://reviews.llvm.org/D4196