Expand includes and defines transitively in 'external' genexes.

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.
Fix constness of accessors.
Fix the tests for evaluating includes and defines.

We should also check whether the INTERFACE_ variant of a property
is being read, and in the case of the compile definitions, we should
test the _<CONFIG> suffixed variants. That is already available
through the use of the methods.

This way, we use the ALREADY_SEEN optimization when evaluating
the includes of a target in 'external' generator expressions, ie, those
used in a add_custom_command invokation, as opposed to evaluating the
INCLUDE_DIRECTORIES of a target itself via GetIncludeDirectories.
Merge topic 'interface-property-external-read' into next

08a9eea Fix the tests for evaluating includes and defines.
880a779 Fix constness of accessors.
a10539b Expand includes and defines transitively in 'external' genexes.