external_project: workaround for MinGW #16973

Job	Run time
Arch Linux	20m 7s
Arch / PyPy	19m 43s
CUDA (on Arch)	7m 10s
Fedora	18m 13s
Ubuntu Rolling (gcc, g++)	19m 52s
Gentoo	22m 3s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 45s
OpenSUSE	16m 26s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 59s
Ubuntu Bionic	15m 2s
Ubuntu Rolling (clang, clang++)	21m 50s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 4s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	12m 56s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 8s
	3h 50m 18s

Provide feedback