msvc: prevent conversion of -L... options #14503

Job	Run time
Arch / PyPy	19m 14s
Arch Linux	16m 30s
CUDA (on Arch)	5m 40s
Fedora	20m 19s
Ubuntu Rolling (gcc, g++)	16m 1s
OpenSUSE	15m 8s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	11m 3s
Ubuntu Bionic	13m 56s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	10m 57s
Ubuntu Rolling (clang, clang++)	19m 4s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	12m 39s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	10m 51s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	10m 16s
	3h 1m 38s

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