Extend the genex added by commit ca6ba3fee5 (Genex: Add a SHELL_PATH
expression, 2015-09-24, v3.4.0-rc1~37^2) to accept a `;`-list of paths,
convert them all, and generate a list separated by the native shell
`PATH``` separator.
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.
