This allows a single condition to be used to choose between two
alternatives. Without this the condition must be duplicated with
one surrounded by `NOT`.
Closes: #15585
Some commands on Windows do not understand forward slash paths and
require backslashes. In order to help projects generate shell
invocations of such commands, provide a generator expression to convert
paths to the shell-preferred path format for the current generator.
This will allow custom commands to generate paths the same way CMake
does for compiler command invocations.