external_project: workaround for MinGW #16978

Job	Run time
Arch Linux	19m 59s
Arch / PyPy	20m 2s
CUDA (on Arch)	7m 8s
Fedora	17m 43s
Gentoo	21m 43s
Ubuntu Rolling (gcc, g++)	19m 33s
OpenSUSE	16m 52s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 28s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 26s
Ubuntu Bionic	15m 32s
Ubuntu Rolling (clang, clang++)	22m 13s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	14m 55s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 53s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 1s
	3h 50m 28s

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