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Fix libc++ headers for older macOS SDKs #126

Job	Run time
macos (x86_64, 10.8, MacOSX10.9.sdk)	47s
windows (aarch64)	1m 58s
macos (aarch64, 11, MacOSX11.0.sdk)	1m 4s
windows (x86_64)	2m 5s
ubuntu (x86_64, g++-)	42s
ubuntu (armhf, g++-arm-linux-gnueabihf)	34s
ubuntu (x86, g++-multilib)	43s
ubuntu (ppc64, g++-powerpc64le-linux-gnu)	48s
ubuntu (mingw64, g++-mingw-w64-x86-64)	41s
ubuntu (mingwaarch64, clang)	1m 5s
ubuntu (aarch64, g++-aarch64-linux-gnu)	33s
ubuntu (riscv64, g++-riscv64-linux-gnu)	2m 27s
ubuntu (mingw32, g++-mingw-w64-i686)	45s
	14m 12s

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