VS: Fix '-T version=14.40' under VS 17.10 preview 1

Brad Kingrequested to merge
brad.king/cmake:vs-17.10 into master

VS 17.10 preview 1 comes with toolset v143 version 14.40. This is the first time that the first three digits of the version do not match the toolset name. Add a special case to map version 14.40 back to toolset v143.

Backport: release

Edited by Brad King

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