When evaluating the `$<CONFIG>` genex on an imported target has the
`IMPORTED_CONFIGURATIONS` property set, the current project has a config
that does not match any of those, and no explicit configuration mappings
are defined, fall back to the same configuration as `IMPORTED_LOCATION`.

Fixes: #24222

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.

Fixes: #20951

Instead of having to do $<OR:$<CONFIG:Release>,$<CONFIG:MinSizeRel>>
you can do $<CONFIG:Release,MinSizeRel>

Generator expressions may contain or produce backslashes.

Fixes: #19553

The geneator expression can now be used with static, shared, and
module libraries and executables.

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.

Use MSYS2's `MSYS2_ARG_CONV_EXCL` environment variable to prevent
path conversion where possible.