Xcode backend: better quoting for spaces in HEADER_SEARCH_PATHS #17333

Job	Run time
Arch / PyPy	1s
Arch / PyPy	19m 49s
Arch Linux	1s
Arch Linux	20m 7s
CUDA (on Arch)	1s
CUDA (on Arch)	7m 19s
Ubuntu Rolling (gcc, g++)	1s
Ubuntu Rolling (gcc, g++)	19m 51s
Fedora	1s
Fedora	17m 46s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	1s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 39s
Gentoo	1s
Gentoo	22m 13s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	1s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 41s
OpenSUSE	1s
OpenSUSE	16m 24s
Ubuntu Rolling (clang, clang++)	1s
Ubuntu Rolling (clang, clang++)	22m 30s
Ubuntu Bionic	1s
Ubuntu Bionic	15m 10s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	1s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 4s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	1s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 24s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	1s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 23s
	3h 50m 34s

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