This means that we can use expressions of the form
$<TARGET_PROPERTY:foo,INTERFACE_INCLUDE_DIRECTORIES>
to get a list of the interface include directories of foo, including
those coming from dependencies.
We can't have a test of a target which has a single include directory in
its INCLUDE_DIRECTORIES because the shell on the MSYS platforms transforms
a single include directory to include a prefix, which is not what the test
expects. We test a target with two directories instead as a means to
test a target with no link dependencies.
Stephen Kelly