build-macos: fix C++20 compatibility for Clang13.1

Fixes compilation on Clang13.1 by backporting some C++20
features that aren't available in Clang and fixing
the CMake config. Note that this doesn't yet properly package
the app nor does anything work at this point.

jthread, stop_token, and condition_variable_any impls taken from
josuttis/jthread and appleCompat.h taken from Citra and modified by me.

src/video_core/shader_environment.h

@@ -11,7 +11,11 @@
 #include <memory>
 #include <optional>
 #include <span>
+#ifdef __APPLE__
+#include "common/apple_compat/appleCompat.h"
+#else
 #include <stop_token>
+#endif
 #include <type_traits>
 #include <unordered_map>
 #include <vector>