Since commit ca6ba3fee5 (Genex: Add a SHELL_PATH expression, 2015-09-24,
v3.4.0-rc1~37^2) and commit 21da25d2a8 (Tests: Generalize
GeneratorExpression MSYS path conversion workaround, 2019-03-11,
v3.15.0-rc1~407^2~1) we use a prefix string to prevent the MSYS 1.0
shell from converting absolute paths of the form `/c/...` to
windows-style paths. In the newer `mingw.osdn.io` distribution of MSYS
1.0, the `bash` shell now also converts paths that appear after `:`.
We have no way to add a prefix string in that context, so perform the
test's check of the `SHELL_PATH` genex value at generate time instead.
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.
Brad King