subproject: Fix inheritance of language options #17125

Job	Run time
Arch Linux	19m 37s
Arch / PyPy	20m 7s
CUDA (on Arch)	7m 13s
Fedora	18m 22s
Ubuntu Rolling (gcc, g++)	18m 35s
Gentoo	22m 6s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 59s
OpenSUSE	17m 1s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 12s
Ubuntu Bionic	15m 17s
Ubuntu Rolling (clang, clang++)	21m 36s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 35s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 2s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 51s
	3h 51m 33s

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