external_project: workaround for MinGW #16979

Job	Run time
Arch / PyPy	19m 59s
Arch Linux	20m 6s
Ubuntu Rolling (gcc, g++)	19m 23s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	15m 31s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 59s
CUDA (on Arch)	7m 4s
Ubuntu Rolling (clang, clang++)	21m 52s
Fedora	18m 10s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 18s
Gentoo	21m 50s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 39s
OpenSUSE	16m 23s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 36s
Ubuntu Bionic	14m 54s
	3h 51m 44s

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