Add C11 threads.h implementation in addition to pthread implementation for multithreaded encoding. #1334

Job	Run time
build (ubuntu-latest-clang-cmake, ubuntu-latest, clang, clang++, cmake)	4m 33s
build (ubuntu-latest-gcc-cmake, ubuntu-latest, gcc, g++, cmake)	6m 19s
build (ubuntu-latest-clang-autotools, ubuntu-latest, clang, clang++, autotools)	6m 24s
build (macos-latest-clang-autotools, macos-latest, clang, clang++, autotools)	8m 38s
build (macos-latest-clang-cmake, macos-latest, clang, clang++, cmake)	7m 0s
build (windows-latest-cmake, windows-latest, cmake)	15m 59s
build (windows-latest-cmake-shared, windows-latest, cmake, -DBUILD_SHARED_LIBS=ON)	15m 58s
build (windows-latest-cmake-c11-shared, windows-latest, cmake, -DCMAKE_C_STANDARD=11 -DBUILD_SHAR...	15m 0s
build (windows-latest-cmake-c11, windows-latest, cmake, -DCMAKE_C_STANDARD=11)	16m 10s
	1h 36m 1s

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