Workaround broken code where a target has itself in its link iface.

There is a test for this since commit 8e756d2b (Tolerate cycles in
shared library link interfaces (#12647), 2012-01-12), so make sure
it continues to pass, even as we require no self-references in new
INTERFACE_ property generator expressions.
Fix DAG checker finding cycling dependencies.

Before this patch, the following is reported falsely as a self-reference:

 target_link_libraries(empty2 LINK_PUBLIC empty3)
 target_link_libraries(empty3 LINK_PUBLIC empty2)

 add_custom_target(...
    -DINCLUDES=$<TARGET_PROPERTY:empty2,INTERFACE_INCLUDE_DIRECTORIES>
 )

The reason is that the existing code assumed that all reading of
include directories would be done through cmTarget::GetIncludeDirectories()
and would therefore be initialized with a DagChecker. That is not the case
if reading the property with an 'external' generator expression.
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

8dfdf1c Fix the tests for evaluating includes and defines.
98a6725 Fix constness of accessors.
7e70744 Expand includes and defines transitively in 'external' genexes.
d1a2729 Fix DAG checker finding cycling dependencies.
e72eaad Workaround broken code where a target has itself in its link iface.