linux

Make all Meson level options overridable per subproject. #17974

Job	Run time
Arch Linux	21m 4s
CUDA (on Arch)	8m 7s
Fedora	21m 15s
Arch / PyPy	21m 39s
Gentoo	25m 27s
Ubuntu Rolling (gcc, g++)	22m 19s
OpenSUSE	19m 14s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	15m 25s
Ubuntu Bionic	15m 48s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	16m 50s
Ubuntu Rolling (clang, clang++)	26m 1s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	18m 16s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	15m 14s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	15m 12s
	4h 21m 51s

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