Prevent msys2 conversion of "cmd /c" switch to a file path

Modern versions of msys2 have changed the treatment of "cmd /c" so that
the runtime will try to convert the switch to a native file path. This
patch adds a setting to inhibit that behaviour.

Discussion: https://postgr.es/m/3227042f-cfcc-745a-57dd-fb8c471f8ddf@2ndQuadrant.com

Backpatch to all live branches.