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.

externals/CMakeLists.txt

@@ -127,3 +127,8 @@ if (YUZU_USE_BUNDLED_FFMPEG)
     set(FFmpeg_LIBRARIES "${FFmpeg_LIBRARIES}" PARENT_SCOPE)
     set(FFmpeg_INCLUDE_DIR "${FFmpeg_INCLUDE_DIR}" PARENT_SCOPE)
 endif()
+
+if (APPLE)
+    add_library(range_v3 INTERFACE)
+    set_target_properties(range_v3 PROPERTIES INTERFACE_INCLUDE_DIRECTORIES ${PROJECT_SOURCE_DIR}/externals/range-v3/include)
+endif()

externals/ffmpeg/CMakeLists.txt

@@ -137,6 +137,7 @@ if (NOT WIN32)
                 --disable-network
                 --disable-postproc
                 --disable-swresample
+                --disable-iconv
                 --enable-decoder=h264
                 --enable-decoder=vp8
                 --enable-decoder=vp9