Xcode backend: only use found appleframeworksn #16908

Job	Run time
Arch Linux	20m 6s
CUDA (on Arch)	7m 37s
Arch / PyPy	20m 59s
Fedora	17m 52s
Gentoo	22m 46s
Ubuntu Rolling (gcc, g++)	19m 44s
OpenSUSE	16m 53s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	14m 6s
Ubuntu Bionic	15m 17s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 7s
Ubuntu Rolling (clang, clang++)	22m 25s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 19s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 41s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 23s
	3h 55m 15s

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