Summary:
Allow a Facebook-specific vcvarsall to be used. The getdeps default of
MSVC 2017 had at least one annoying bug fixed in 2019.

Reviewed By: chadaustin

Differential Revision: D31188039

fbshipit-source-id: 1f569fe1c5f4ceb68f0ddd339e6de3c8ca07588b