linux

cargo: Fix crash when enabling feature on dev/build dependencies #35

Job	Run time
Arch Linux	19m 48s
CUDA (on Arch)	7m 21s
Arch / PyPy	19m 35s
Fedora	17m 56s
Gentoo	21m 50s
Ubuntu Rolling (gcc, g++)	19m 44s
OpenSUSE	16m 32s
Ubuntu Bionic	48s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 42s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 22s
Ubuntu Rolling (clang, clang++)	22m 5s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 25s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 38s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 53s
	3h 36m 39s

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