external_project: workaround for MinGW #16976

Job	Run time
Arch Linux	20m 2s
Arch / PyPy	19m 20s
CUDA (on Arch)	7m 25s
Fedora	18m 8s
Gentoo	22m 23s
Ubuntu Rolling (gcc, g++)	19m 44s
OpenSUSE	16m 38s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 36s
Ubuntu Bionic	15m 0s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 52s
Ubuntu Rolling (clang, clang++)	22m 32s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 47s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 49s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 38s
	3h 51m 54s

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