Justin Goshi authored Jun 02, 2020 and Brad King committed Jun 03, 2020

VS 16.6 added a `StdOutEncoding` setting for custom commands to tell
MSBuild that the output is encoded as UTF-8.  In commit bc877a7e (Add
support to indicate UTF-8 custom command pipe output encoding,
2020-04-08) CMake learned to add the setting in anticipation of the VS
16.6 release.  However, when 16.6 was released it had a bug in the
implementation of custom tasks with StdOutEncoding enabled that was
exposed by our test suite.  In commit 5058fb54 (VS: Drop
StdOutEncoding with VS 16.6 pending investigation, 2020-05-29) we
disabled the setting pending investigation.

The problem is fixed in VS 16.7 Preview 3, so restore use of the
setting when a VS instance of at least that version is detected.

Fixes: #20769