This adds generic APIs to libqb for handling thread's, mutex's, and
condition variables. On Linux and OSX these are implemented via the ones
provided by pthreads. On Windows they're implemented via the ones
provided by the Win32 API.
For compiling, the code itself is not conditional, but the Makefile
includes logic to decide which implementation to pick.
Note that it would have been nice to simply use std::thread and friends
from C++11, however using them on MinGW appears to be a bit messy. Since
using the Windows ones directly isn't that hard this was an easy compromise.
This sets up a few different flags we'll need for the conditional
compiling, and also sets the C++ standard to gnu++11, which effectively
just matches what we were implicitly using before.
