external_project: workaround for MinGW #16981

Job	Run time
Arch / PyPy	10m 20s
Arch Linux	10m 18s
Ubuntu Rolling (gcc, g++)	7m 49s
CUDA (on Arch)	7m 9s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	10m 6s
Fedora	10m 19s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	10m 7s
Gentoo	10m 18s
Ubuntu Rolling (clang, clang++)	8m 45s
OpenSUSE	10m 18s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	10m 4s
Ubuntu Bionic	10m 9s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	10m 7s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	10m 9s
	2h 15m 58s

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