Since commit ca6ba3fe (Genex: Add a SHELL_PATH expression, 2015-09-24,
v3.4.0-rc1~37^2) and commit 21da25d2 (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.