PyTorch 1.6.0 has updated the vendored pthreadpool library, which has recently
added support for Grand Central Dispatch. Unfortunately, it uses functionality
(DISPATCH_APPLY_AUTO) that is only available since macOS 10.13, whereas we are
still using 10.12 libraries.
We can't directly pass through option to vendored libraries, since the setup.py
scripts creates/filters the options that are passed to CMake. So, instead, this
adds a small patch that disables the GCD functionality in pthreadpool.
